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Share-Based Payments - Summary of Grants, Exchanges and Forfeitures of Restricted Stock and RSUs (Details) - LTIP [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Estimate Fair Value on Grant Date $ 17,496 $ 16,477 $ 12,769
Restricted Stock [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Outstanding, beginning balance (in shares) 150,110 107,385 73,927
Grant (in shares) 27,700 92,056 65,950
Vested (in shares) (72,249) (49,331) (32,492)
Forfeiture (in shares) 0 0 0
Outstanding, ending balance (in shares) 105,561 150,110 107,385
RSU [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Outstanding, beginning balance (in shares) 1,703,775 1,242,810 610,254
Grant (in shares) 912,916 903,068 666,056
Vested (in shares) (938,541) (397,030) (20,000)
Forfeiture (in shares) (45,404) (45,073) (13,500)
Outstanding, ending balance (in shares) 1,632,746 1,703,775 1,242,810