Financial Instruments and Fair Values - Gain and Loss of Cash Flow Hedges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ 68,473	$ 70,595	$ 65,743
Cash Flow Hedging [Member] | Designated as Hedging Instrument [Member] | Interest rate swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in other comprehensive income (loss)	(11,658)	(3,054)	(1,922)
Amount of gain (loss) reclassified from accumulated other comprehensive income (loss) into interest expense	(1,142)	0	0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Interest rate swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Interest expense	$ 1,142	$ 0	$ 0