Fair Value (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013
Derivative instruments and Hedging Activities
Hedging losses reclassified from accumulated other comprehensive income into earnings	$ 1,438		$ 1,574	$ 3,011		$ 2,965
Non-Designated Hedges
Derivative instruments and Hedging Activities
Notional balance	609,960			609,960
Weighted average interest rate (as a percent)	1.70%	[1]		1.70%	[1]
Weighted average capped interest rate (as a percent)	5.90%			5.90%
Number of derivative instruments held	14			14
Unrecognized gain on derivatives			1,069			2,484
Cash Flow Hedges
Derivative instruments and Hedging Activities
Notional balance	171,691			171,691
Weighted average interest rate (as a percent)	2.00%	[1]		2.00%	[1]
Weighted average capped interest rate (as a percent)	5.10%			5.10%
Number of derivative instruments held	4			4
Estimated hedging losses to be reclassified from accumulated other comprehensive loss into earnings within the next twelve months				$ 5,493

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