Derivative and Other Fair Value Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Increase (Decrease) in Fair Value of Interest Rate Fair Value Hedging Instruments	$ (4,663)	$ (727)
Increase (Decrease) in Fair Value of Hedged Item in Interest Rate Fair Value Hedge	4,663	727
Effective Portion - Amount of Gain/ (Loss) Recognized in OCI on Derivative	(8,642)	(25,119)
Effective Portion - Amount of Gain/ (Loss) Reclassified from Accumulated OCI into Income	(7,203)	(1,891)
Ineffective Portion - Amount of Gain/ (Loss) Reclassifed from Accumulated OCI into Income	0	(2,569)
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Increase (Decrease) in Fair Value of Interest Rate Fair Value Hedging Instruments	(4,663)	(727)
Increase (Decrease) in Fair Value of Hedged Item in Interest Rate Fair Value Hedge	4,663	727
Derivative, Description of Hedged Item	Fixed rate debt	Fixed rate debt
Forward Starting Swaps/Treasury Locks [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective Portion - Amount of Gain/ (Loss) Recognized in OCI on Derivative	(8,642)	(26,441)
Effective Portion - Amount of Gain/ (Loss) Reclassified from Accumulated OCI into Income	(7,203)	(1,891)
Ineffective Portion - Amount of Gain/ (Loss) Reclassifed from Accumulated OCI into Income	0	(2,569)
Development Interest Rate Swaps Caps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective Portion - Amount of Gain/ (Loss) Recognized in OCI on Derivative		1,322
Effective Portion - Amount of Gain/ (Loss) Reclassified from Accumulated OCI into Income		0
Ineffective Portion - Amount of Gain/ (Loss) Reclassifed from Accumulated OCI into Income		$ 0