Financial Instruments: Derivatives and Hedging - Interest Rate Swaps (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amount of Interest Expense presented in the consolidated statements of operations	$ 36,026	$ 36,757	$ 38,374
Interest rate swap
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amount of Interest Expense presented in the consolidated statements of operations	36,026	36,757	38,374
Interest rate swap | Amounts of previously recorded gain/(loss) reclassified from OCI into Interest Expense
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amount of Interest Expense presented in the consolidated statements of operations	(7,751)	4,328	2,722
Interest rate swap | Amounts of gain (loss) recognized in OCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amount of Interest Expense presented in the consolidated statements of operations	$ (20,380)	$ (15,119)	$ 5,321