customers are included in revenues, and represent less than one percent of total revenues. The corresponding sales taxes paid  are included in cost of goods sold. Value added taxes collected from customers in international jurisdictions are recorded in  accrued expenses as a liability. Revenue is recorded net of discounts. Sales returns are accrued based on historical sales return  experience, which we believe provides a reasonable estimate of future returns. A portion of Enterprise Networks products are sold to a non-exclusive distribution network of major technology distributors in the United States. These large organizations then distribute or provide fulfillment services to an extensive network of value-added resellers and system integrators. Value-added resellers and system integrators may be affiliated with us as a channel partner, or they may purchase from the distributor in an unaffiliated fashion. Additionally, with certain limitations our distributors may return unused and unopened product for stock-balancing purposes when such returns are  accompanied by offsetting orders for products of equal or greater value. We participate in cooperative advertising and market development programs with certain customers. We use these programs to reimburse customers for certain forms of advertising, and in general, to allow our customers credits up to a specified percentage of their net purchases. Our costs associated with these programs are estimated and included in marketing expenses in our consolidated statements of income. We also participate in rebate programs to provide sales incentives for certain products. Our costs associated with these programs are estimated and accrued at the time of sale, and are recorded as a reduction of sales in our consolidated statements of income. Unearned revenue primarily represents customer billings on our maintenance service programs and unearned revenues relating to multiple element contracts where we still have contractual obligations to our customers. We currently offer maintenance contracts ranging from one to five years, primarily on Enterprise Networks Division products sold through  distribution channels. Revenue attributable to maintenance contracts is recognized on a straight-line basis over the related  contract term. In addition, we provide software maintenance and a variety of hardware maintenance services to Carrier Networks Division customers, which include customers of the acquired NSN BBA business, under contracts with terms up to ten years. At December 31, 2013 and 2012, unearned revenue was as follows: Other income (expense), net, is comprised primarily of miscellaneous income and expense, gains and losses on foreign currency transactions, investment account management fees, and gains or losses on the disposal of property, plant and equipment occurring in the normal course of business. Earnings per common share, and earnings per common share assuming dilution, are based on the weighted average number  of common shares and, when dilutive, common equivalent shares outstanding during the year. See Note 14 of Notes to Consolidated Financial Statements for additional information. The Board of Directors presently anticipates that it will declare a regular quarterly dividend as long as the current tax treatment  of dividends exists and adequate levels of liquidity are maintained. During the years ended December 31, 2013, 2012 and 2011,  we paid $21.4 million, $22.8 million and $23.1 million, respectively, in shareholder dividends. On January 21, 2014, the Board  of Directors declared a quarterly cash dividend of $0.09 per common share to be paid to shareholders of record at the close of  business on February 6, 2014. The ex-dividend date was February 4, 2014 and the payment date was February 20, 2014. The  quarterly dividend payment was $5.1 million. We use the acquisition method to account for business combinations. Under the acquisition method of accounting, we recognize the assets acquired and liabilities assumed at their fair value on the acquisition date. Goodwill is measured as the  excess of the consideration transferred over the net assets acquired. Costs incurred to complete the business combination, such as legal, accounting or other professional fees, are charged to general and administrative expenses as they are incurred. ADTRAN 2013 Annual Report 46 Unearned Revenue Other Income (Expense), Net Earnings per Share Dividends Business Combinations (In thousands) 2013 2012 Current unearned revenue $22,205 $35,326 Non-current unearned revenue 14,643 22,377 Total $36,848 $57,703