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Derivative Instruments and Hedging Activities - Derivative Instruments Included in the Company's Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated other comprehensive income (loss) for derivative instruments:      
Balance at beginning of the period $ 848,075    
Balance at end of the period 971,865 $ 848,075  
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent      
Accumulated other comprehensive income (loss) for derivative instruments:      
Balance at beginning of the period 102 304 $ 1,135
Unrealized loss on interest rate swaps (102) (202) (831)
Balance at end of the period $ 0 $ 102 $ 304