Table of Contents Item 6. Selected Financial Data As of and for the following periods (in thousands except for share and per share data): Years Ended December 31, 2019 2018 (a) 2017 2016 2015 Operating revenue $ 861,994 $ 903,603 $ 916,797 $ 886,936 $ 849,869 Operating income (b) $ 178,093 $ 172,229 $ 190,059 $ 187,911 $ 146,191 Net income $ 123,037 $ 84,315 $ 98,261 $ 96,768 $ 81,918 Basic earnings per share: Net income $ 3.02 $ 2.07 $ 2.42 $ 2.39 $ 2.03 Weighted average number of shares outstanding 40,605,597 40,521,364 40,414,556 40,350,688 40,274,986 Diluted earnings per share: Net income $ 3.01 $ 2.07 $ 2.42 $ 2.39 $ 2.03 Weighted average number of shares and dilutive  potential shares outstanding 40,681,221 40,642,640 40,535,191 40,408,033 40,308,562 Dividends declared per share of common stock $ 1.515 $ 1.415 $ 1.315 $ 1.225 $ 1.165 Cash additions to utility property, plant and equipment (c) $ 222,203 $ 240,021 $ 199,896 $ 229,722 $ 281,458 Total assets $ 3,813,200 $ 3,628,502 $ 3,484,363 $ 3,376,278 $ 3,200,607 Long-term debt, net of current portion $ 1,340,981 $ 1,285,980 $ 1,195,988 $ 1,195,513 $ 1,122,660 Common stock equity $ 1,216,547 $ 1,164,103 $ 1,142,165 $ 1,074,396 $ 1,016,538 24 (a) Effective January 1, 2018, the Company implemented Accounting Standards Update ("ASU") 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Liabilities. As required by the new standard, changes in the fair values of the Company's equity investments are recognized in earnings, whereas prior to 2018, such changes were recognized in accumulated other comprehensive income ("AOCI"). (b) The Company implemented ASU 2017-07, Compensation - Retirement Benefits (Topic 715), in the first quarter of 2018, and as required by the standard, reclassified certain amounts in the financial statements for 2017 and 2016. (c) The Company implemented ASU 2016-15, Statement of Cash Flows (Topic 230), in the first quarter of 2018, and as required by the standard, reclassified certain amounts in the financial statements for 2017 and 2016.