Fair Value (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Derivative instruments and Hedging Activities
Hedging losses reclassified from accumulated other comprehensive income into earnings	$ 1,565		$ 1,573
Estimated hedging losses to be reclassified from accumulated other comprehensive loss into earnings within the next twelve months	5,493
Not Designated as Hedging Instrument [Member]
Derivative instruments and Hedging Activities
Number of derivative instruments held	13
Cash Flow Hedges
Derivative instruments and Hedging Activities
Weighted average interest rate (as a percent)	2.50%	[1]
Number of derivative instruments held	4
Interest Rate Cap [Member] | Cash Flow Hedges
Derivative instruments and Hedging Activities
Notional balance	170,512
Weighted average capped interest rate (as a percent)	5.10%
Not Designated as Hedging Instrument [Member] | Interest Rate Cap [Member]
Derivative instruments and Hedging Activities
Notional balance	$ 695,067
Weighted average interest rate (as a percent)	1.90%	[1]
Weighted average capped interest rate (as a percent)	6.20%

[1] Represents the weighted average interest rate on the hedged debt.