Derivative and Other Fair Value Instruments (Details 1) (Designated as Hedging Instrument [Member], USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Schedule of Location of Derivatives on Balance Sheet
Derivative asset, fair value	$ 13,823	$ 15,797
Derivative liability, fair value	65,519	39,078
Fair Value Hedges [Member] | Other Assets [Member]
Schedule of Location of Derivatives on Balance Sheet
Derivative asset, fair value	11,794	12,521
Fair Value Hedges [Member] | Other Liabilities [Member]
Schedule of Location of Derivatives on Balance Sheet
Derivative liability, fair value	0	0
Forward Starting Swaps [Member] | Other Assets [Member]
Schedule of Location of Derivatives on Balance Sheet
Derivative asset, fair value	2,029	3,276
Forward Starting Swaps [Member] | Other Liabilities [Member]
Schedule of Location of Derivatives on Balance Sheet
Derivative liability, fair value	65,519	37,756
Development Cash Flow Hedges [Member] | Other Assets [Member]
Schedule of Location of Derivatives on Balance Sheet
Derivative asset, fair value		0
Development Cash Flow Hedges [Member] | Other Liabilities [Member]
Schedule of Location of Derivatives on Balance Sheet
Derivative liability, fair value		$ 1,322