(20) Lease Obligations (21) Commitments, Litigation and Contingent Liabilities (22) Stock Incentive Plan Upon vesting, shares of the Company’s common stock equal to the number of vested RSUs are issued or deferred to a later date, depending on the terms of the specific award agreement. As of December 31, 2018, 4,709,318 RSUs had been deferred. RSUs that have not yet vested and vested RSUs that have been deferred are not considered to be issued and outstanding shares. The fair value of RSUs at the date of grant are recorded as unearned compensation, a component of stockholders’ equity, and expensed over the vesting period. Following is a summary of changes in unearned compensation for the three years ended December 31, 2018: Pursuant to the Company's stock incentive plan, the Company may issue restricted stock units ("RSUs") to employees of the Company and its subsidiaries. The RSUs generally vest three to five years from the award date and are subject to other vesting and forfeiture provisions contained in the award agreement. The following table summarizes RSU information for the three years ended December 31, 2018: At December 31, 2018, the Company had commitments to invest up to $270.2 million and $253.4 million in certain investment funds and real estate construction projects, respectively. In the ordinary course of business, the Company is subject to disputes, litigation and arbitration arising from its insurance and reinsurance businesses. These matters are generally related to insurance and reinsurance claims and are considered in the establishment of loss and loss expense reserves. In addition, the Company may also become involved in legal actions which seek extra-contractual damages, punitive damages or penalties, including claims alleging bad faith in handling of insurance claims. The Company expects its ultimate liability with respect to such matters will not be material to its financial condition. However, adverse outcomes on such matters are possible, from time to time, and could be material to the Company’s results of operations in any particular financial reporting period. The Company and its subsidiaries use office space and equipment under leases expiring at various dates. These leases are considered operating leases for financial reporting purposes. Some of these leases have options to extend the length of the leases and contain clauses for cost of living, operating expense and real estate tax adjustments. Future minimum lease payments, without provision for sublease income, are: $46,592,000 in 2019; $43,504,000 in 2020; $39,061,000 in 2021; $34,444,000 in 2022, $30,881,000 in 2023 and $75,740,000 thereafter. Rental expense was $45,778,000, $52,925,000, and $47,453,000 for 2018, 2017, and 2016 respectively. 2018 2017 2016 RSUs granted and unvested at beginning of period: 3,477,981 4,862,098 4,158,325 Granted 760,032 855,984 1,000,559 Vested (600,169 ) (1,993,507 ) (77,250 ) Canceled (263,411) (246,594) (219,536) RSUs granted and unvested at end of period: 3,374,433 3,477,981 4,862,098 (In thousands) 2018 2017 2016 Unearned compensation at beginning of year $ 122,910 $ 115,965 $ 103,538 RSUs granted, net of cancellations 52,204 52,897 52,697 RSUs expensed (34,408 ) (38,796 ) (35,585 ) RSUs forfeitures (11,037) (7,156) (4,685) Unearned compensation at end of year $ 129,669 $ 122,910 $ 115,965 99