Derivative Financial Instruments - Impact on Interest Expense of Interest Rate Agreements Accounted for as Cash Flow Hedges (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Total interest expense	$ (36,108)	$ (27,443)	$ (24,234)	$ (21,031)	$ (20,537)	$ (21,597)	$ (22,444)	$ (23,950)	$ (108,816)	$ (88,528)	$ (95,883)
Cash Flow Hedging | Interest Rate Contract
Derivative [Line Items]
Net interest expense on cash flows hedges									(4,602)	(5,317)	(5,883)
Realized net losses reclassified from other comprehensive income									(45)	(72)	(95)
Total interest expense									$ (4,647)	$ (5,389)	$ (5,978)