Derivative and Other Hedging Instruments (Remaining Interest Rate Swap Term) (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Beginning notional amount		$ 6,450,000,000	$ 2,050,000,000	$ 650,000,000
Average fixed pay rate		1.63%
Average receive rate		0.26%
Net Estimated Fair Value		(37,722,000)
Average Maturity (Years)		3.1
Notional amount of forward starting swaps	2,600,000,000
Remaining interest rate swap term, minimum	one
Remaining interest rate swap term, maximum	five
1 Year Or Less [Member]
Beginning notional amount		750,000,000
Average fixed pay rate		1.40%
Average receive rate		0.26%
Net Estimated Fair Value		(5,595,000)
Average Maturity (Years)		0.7
Greater Than 1 Year And Less Than 3 Years [Member]
Beginning notional amount		2,850,000,000
Average fixed pay rate		1.54%
Average receive rate		0.26%
Net Estimated Fair Value		(32,865,000)
Average Maturity (Years)		2.5
Greater Than 3 Years And Less Than 5 Years [Member]
Beginning notional amount		2,850,000,000
Average fixed pay rate		1.78%
Average receive rate		0.26%
Net Estimated Fair Value		738,000
Average Maturity (Years)		4.3
Not Designated as Hedging Instrument [Member]
Beginning notional amount	30,250,000,000
Average Maturity (Years)	3.5
Average Fixed Pay Rate	1.57%
Average Receive Rate	0.35%
Net Estimated Fair Value	(782,512,000)
Greater Than One Year and Less Than or Equal to Three Years [Member]
Beginning notional amount	11,350,000,000
Average Maturity (Years)	2.1
Average Fixed Pay Rate	1.22%
Average Receive Rate	0.30%
Net Estimated Fair Value	(148,797,000)
Greater Than Three Years and Less Than or Equal to Five Years [Member]
Beginning notional amount	16,700,000,000
Average Maturity (Years)	3.9
Average Fixed Pay Rate	1.77%
Average Receive Rate	0.35%
Net Estimated Fair Value	(606,651,000)
Greater Than Five Years and Less than or Equal to Seven Years [Member]
Beginning notional amount	950,000,000
Average Maturity (Years)	5.7
Average Fixed Pay Rate	1.56%
Average Receive Rate	0.57%
Net Estimated Fair Value	(8,690,000)
Greater Than Seven Years and Less than or Equal to Ten Years [Member]
Beginning notional amount	1,250,000,000
Average Maturity (Years)	8.2
Average Fixed Pay Rate	1.99%
Average Receive Rate	0.55%
Net Estimated Fair Value	$ (18,374,000)