Derivative and Other Fair Value Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Effective Portion - Amount of Gain/ (Loss) Recognized in OCI on Derivative	$ 3,218	$ 6,082
Effective Portion - Amount of Gain/ (Loss) Reclassified from Accumulated OCI into Income	(3,563)	(956)
Ineffective Portion - Amount of Gain/ (Loss) Reclassifed from Accumulated OCI into Income	0	0
Forward Starting Swaps/Treasury Locks [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective Portion - Amount of Gain/ (Loss) Recognized in OCI on Derivative	3,218	5,255
Effective Portion - Amount of Gain/ (Loss) Reclassified from Accumulated OCI into Income	(3,563)	(956)
Ineffective Portion - Amount of Gain/ (Loss) Reclassifed from Accumulated OCI into Income	0	0
Development Interest Rate Swaps Caps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective Portion - Amount of Gain/ (Loss) Recognized in OCI on Derivative		827
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