Financial Instruments and Fair Values - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) derivative	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) derivative	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements liability	$ 0		$ 0		$ 436,000
Unrealized gain (loss) on valuation of interest rate swap agreements	1,763,000	$ (14,410,000)	5,943,000	$ (13,016,000)
Loss from derivative financial instruments	0	42,000	0	289,000
Designated as Hedging Instrument | Cash Flow Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred losses on terminated hedges	$ 14,400,000		14,400,000		$ 15,100,000
Loss expected to be reclassified within twelve months			$ 1,500,000
Loss from derivative financial instruments		40,000		300,000
Notional amount, terminated		20,000,000		20,000,000
Designated as Hedging Instrument | Cash Flow Hedging | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, number of instruments held | derivative	1		1		1
Notional amount	$ 265,000,000		$ 265,000,000		$ 265,000,000.0
Unrealized gain (loss) on valuation of interest rate swap agreements	2,300,000	$ (14,400,000)	7,100,000	$ (13,000,000)
Amount expected to be reclassified within twelve months			(600,000)
Prepaid expenses and other assets | Designated as Hedging Instrument | Cash Flow Hedging | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements asset	$ 6,000,000		$ 6,000,000
Accounts payable and accrued expenses | Designated as Hedging Instrument | Cash Flow Hedging | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements liability					$ 400,000