Financial Instruments and Fair Values - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) derivative	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) derivative	Sep. 30, 2014 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2014 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements liability	$ 4,384		$ 4,384			$ 0
Other comprehensive income (loss) unrealized loss on valuation of swap agreement	(4,384)	$ 0	(4,384)	$ 0
Interest expense	16,680	$ 17,674	50,298	$ 46,640
Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements liability	4,600		4,600
Designated as Hedging Instrument | Cash Flow Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other comprehensive income (loss) unrealized loss on valuation of swap agreement	4,400
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	$ 4,400		$ 4,400
Derivative, number of instruments held (derivative) | derivative	3		3
Derivative, notional amount	$ 465,000		$ 465,000
Designated as Hedging Instrument | Cash Flow Hedging | Interest Rate Swap | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap agreements liability	$ 4,400		$ 4,400
Minimum | Designated as Hedging Instrument | Cash Flow Hedging | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, fixed interest rate	2.1485%		2.1485%
Maximum | Designated as Hedging Instrument | Cash Flow Hedging | Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, fixed interest rate	2.505%		2.505%
Estimate | Reclassification out of Accumulated Other Comprehensive Income | Designated as Hedging Instrument | Cash Flow Hedging
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest expense					$ 0