Derivatives and Hedging (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Financial Instruments: Derivatives and Hedging
Unrealized gains (losses) included in accumulated other comprehensive loss	$ 23,047,000		$ 13,644,000
Amount of ineffective hedges	0	0
Interest rate swap
Financial Instruments: Derivatives and Hedging
Interest rate swap liability	(23,000,000)		(13,600,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type one
Financial Instruments: Derivatives and Hedging
Notional value	275,000,000		275,000,000
Hedge interest rate (as a percent)	1.1175%
Interest rate swap liability	(1,942,000)		(232,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type two
Financial Instruments: Derivatives and Hedging
Notional value	175,000,000		175,000,000
Hedge interest rate (as a percent)	1.5625%
Interest rate swap liability	(3,307,000)		(2,182,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type three
Financial Instruments: Derivatives and Hedging
Notional value	175,000,000		175,000,000
Hedge interest rate (as a percent)	1.635%
Interest rate swap liability	(3,691,000)		(2,596,000)
Designated as Hedging Instrument | ISwap-cash flow, hedge type four
Financial Instruments: Derivatives and Hedging
Notional value	16,500,000		16,500,000
Hedge interest rate (as a percent)	1.825%
Interest rate swap liability	(441,000)		(315,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type five
Financial Instruments: Derivatives and Hedging
Notional value	16,500,000		16,500,000
Hedge interest rate (as a percent)	1.751%
Interest rate swap liability	(399,000)		(270,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type six
Financial Instruments: Derivatives and Hedging
Notional value	40,500,000		40,500,000
Hedge interest rate (as a percent)	1.825%
Interest rate swap liability	(1,083,000)		(772,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type seven
Financial Instruments: Derivatives and Hedging
Notional value	41,500,000		41,500,000
Hedge interest rate (as a percent)	1.751%
Interest rate swap liability	(1,003,000)		(678,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type eight
Financial Instruments: Derivatives and Hedging
Notional value	18,000,000		18,000,000
Hedge interest rate (as a percent)	1.825%
Interest rate swap liability	(481,000)		(343,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type nine
Financial Instruments: Derivatives and Hedging
Notional value	17,000,000		17,000,000
Hedge interest rate (as a percent)	1.751%
Interest rate swap liability	(411,000)		(278,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type ten
Financial Instruments: Derivatives and Hedging
Notional value	125,000,000		125,000,000
Hedge interest rate (as a percent)	2.018%
Interest rate swap liability	(4,251,000)		(3,073,000)
Designated as Hedging Instrument | Swap-cash flow, hedge type eleven
Financial Instruments: Derivatives and Hedging
Notional value	100,000,000		100,000,000
Hedge interest rate (as a percent)	1.944%
Interest rate swap liability	(3,105,000)		(2,145,000)
Designated as Hedging Instrument | Interest Rate, Swap Hedge, Type Twelve [Member] [Member]
Financial Instruments: Derivatives and Hedging
Notional value	143,000,000		143,000,000
Hedge interest rate (as a percent)	1.81%
Interest rate swap liability	(2,933,000)		(760,000)
Interest Expense
Financial Instruments: Derivatives and Hedging
Amount reclassified from accumulated other comprehensive income into interest expense	$ 4,100,000	$ 2,900,000