Financial Instruments and Fair Values - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements liability	$ 0		$ 0		$ (436,000)
Loss from derivative financial instruments	0	$ 0	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	14,100,000		14,100,000		15,100,000
Loss expected to be reclassified within twelve months			1,500,000
Loss from derivative financial instruments				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]
Notional amount	515,000,000.0		515,000,000.0		265,000,000
Unrealized gain (loss) on valuation of interest rate swap agreements	4,300,000	$ (1,300,000)	11,400,000	$ (14,300,000)
Amount expected to be reclassified within twelve months			(2,300,000)
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	$ 9,900,000		$ 9,900,000
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)