Financial Instruments and Fair Values - Effect of Derivative Financial Instruments Designated as Cash Flow Hedges (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	$ (12,346)	$ 1,763	$ (19,736)	$ 5,943
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(169)	(536)	(318)	(1,135)
Interest expense	(20,597)	(20,525)	(41,286)	(38,116)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	(12,346)	1,763	(19,736)	5,943
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(169)	(536)	(318)	(1,135)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income (loss) | Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ (169)	$ (536)	$ (318)	$ (1,135)