Financial Instruments and Fair Values - Gain and Loss of Cash Flow Hedges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	$ (2,721)	$ (11,658)
Amount of gain (loss) recognized in other comprehensive income (loss)			$ (3,054)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(1,845)	(1,142)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense			0
Interest expense	(79,623)	(68,473)	(70,595)
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	(2,721)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(1,845)
Cash Flow Hedging | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	(900)	(10,500)
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)		(11,658)
Amount of gain (loss) recognized in other comprehensive income (loss)			(3,054)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense		(1,142)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense			0
Accumulated other comprehensive income (loss) | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ (1,845)	$ (1,142)	$ 0