Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Aggregate notional amount and estimated net fair value of derivative instruments
Notional amount	$ 1,590,845	$ 1,606,787
Fair value amount	(166,107)	(180,139)
Interest Rate Cap [Member]
Aggregate notional amount and estimated net fair value of derivative instruments
Notional amount	36,000	36,000
Fair value amount	997	1,360
Interest rate swap [Member]
Aggregate notional amount and estimated net fair value of derivative instruments
Notional amount	1,554,845	1,570,787
Fair value amount	$ (167,104)	$ (181,499)