Financial Instruments and Fair Values - Gain and Loss of Cash Flow Hedges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	$ (19,322)	$ (21,813)	$ (2,721)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(8,870)	(1,231)	(1,845)
Interest expense	(89,907)	(79,246)	(79,623)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	(19,322)	(21,813)	(2,721)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(8,870)	(1,231)	(1,845)
Accumulated other comprehensive income (loss) | Interest rate swaps | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ (8,870)	$ (1,231)	$ (1,845)