Financial Instruments and Fair Values - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Interest rate swap agreements liability	$ 436	$ 5,591
Unrealized loss on valuation of interest rate swap agreements	11,658	3,054	$ 1,922
Interest rate swaps [Member]
Derivative [Line Items]
Interest rate swap agreements liability	500
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Unrealized loss on valuation of interest rate swap agreements	10,500	3,100
Loss expected to be reclassified within twelve months	1,000
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest rate swaps [Member]
Derivative [Line Items]
Notional amount	265,000	890,000
Prepaid expenses and other assets [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest rate swaps [Member]
Derivative [Line Items]
Interest rate swap agreements asset	600
Accounts payable and accrued expenses [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest rate swaps [Member]
Derivative [Line Items]
Interest rate swap agreements liability	$ 400	$ 5,600