Financial Instruments: Derivatives and Hedging - Interest Rate Swaps (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amount of Interest Expense presented in the consolidated statements of operations	$ 8,980	$ 9,371	$ 18,043	$ 18,739
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			18,043	18,739
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			(2,163)	2,930
Interest rate swap | Amounts of 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			$ (20,439)	$ (15,322)