Derivative Instruments Notional and FV Position (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Asset Derivatives [Abstract]
Notional Amount	$ 396,866,000,000		$ 336,877,000,000
Estimated Fair Value	435,000,000		561,000,000
Derivative Liabilities [Abstract]
Notional Amount	464,145,000,000		377,748,000,000
Estimated Fair Value	(705,000,000)		(916,000,000)
Table Footnote [Abstract]
Cash collateral posted	6,300,000,000		6,800,000,000
Cash collateral received	0		779,000,000
Risk Management Derivatives [Member]
Asset Derivatives [Abstract]
Notional Amount	331,075,000,000		293,090,000,000
Estimated Fair Value	14,828,000,000		15,085,000,000
Accrued interest receivable	1,242,000,000		920,000,000
Netting adjustment	(15,791,000,000)	[1]	(15,829,000,000)	[1]
Total net risk management derivatives	279,000,000		176,000,000
Derivative Liabilities [Abstract]
Notional Amount	370,879,000,000		320,100,000,000
Estimated Fair Value	(20,946,000,000)		(21,022,000,000)
Accrued interest payable	(1,508,000,000)		(1,238,000,000)
Netting adjustment	22,046,000,000	[1]	21,898,000,000	[1]
Total net risk management derivatives	(408,000,000)		(362,000,000)
Risk Management Derivatives [Member] | Pay-fixed Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	19,450,000,000		30,950,000,000
Estimated Fair Value	270,000,000		102,000,000
Derivative Liabilities [Abstract]
Notional Amount	239,017,000,000		155,807,000,000
Estimated Fair Value	(18,237,000,000)		(17,391,000,000)
Risk Management Derivatives [Member] | Receive-fixed Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	231,346,000,000		170,668,000,000
Estimated Fair Value	10,514,000,000		8,118,000,000
Derivative Liabilities [Abstract]
Notional Amount	57,190,000,000		59,027,000,000
Estimated Fair Value	(200,000,000)		(93,000,000)
Risk Management Derivatives [Member] | Basis Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	23,199,000,000		382,000,000
Estimated Fair Value	151,000,000		122,000,000
Derivative Liabilities [Abstract]
Notional Amount	1,700,000,000		9,240,000,000
Estimated Fair Value	0		(44,000,000)
Risk Management Derivatives [Member] | Foreign Currency Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	686,000,000		581,000,000
Estimated Fair Value	193,000,000		155,000,000
Derivative Liabilities [Abstract]
Notional Amount	509,000,000		451,000,000
Estimated Fair Value	(45,000,000)		(62,000,000)
Risk Management Derivatives [Member] | Pay-fixed Swaption [Member]
Asset Derivatives [Abstract]
Notional Amount	33,050,000,000		48,600,000,000
Estimated Fair Value	102,000,000		165,000,000
Derivative Liabilities [Abstract]
Notional Amount	36,225,000,000		47,750,000,000
Estimated Fair Value	(184,000,000)		(194,000,000)
Risk Management Derivatives [Member] | Receive-fixed Swaption [Member]
Asset Derivatives [Abstract]
Notional Amount	15,970,000,000		33,695,000,000
Estimated Fair Value	3,572,000,000		6,371,000,000
Derivative Liabilities [Abstract]
Notional Amount	36,225,000,000		47,750,000,000
Estimated Fair Value	(2,279,000,000)		(3,238,000,000)
Risk Management Derivatives [Member] | Other [Member]
Asset Derivatives [Abstract]
Notional Amount	7,374,000,000	[2]	8,214,000,000	[2]
Estimated Fair Value	26,000,000	[2]	52,000,000	[2]
Derivative Liabilities [Abstract]
Notional Amount	13,000,000	[2]	75,000,000	[2]
Estimated Fair Value	(1,000,000)	[2]	0	[2]
Mortgage Commitment Derivatives [Member]
Asset Derivatives [Abstract]
Notional Amount	65,791,000,000		43,787,000,000
Estimated Fair Value	156,000,000		385,000,000
Derivative Liabilities [Abstract]
Notional Amount	93,266,000,000		57,648,000,000
Estimated Fair Value	(297,000,000)		(554,000,000)
Mortgage Commitment Derivatives [Member] | Mortgage Commitments to Purchase Whole Loans [Member]
Asset Derivatives [Abstract]
Notional Amount	12,360,000,000		9,710,000,000
Estimated Fair Value	27,000,000		73,000,000
Derivative Liabilities [Abstract]
Notional Amount	5,232,000,000		422,000,000
Estimated Fair Value	(8,000,000)		0
Mortgage Commitment Derivatives [Member] | Forward Contracts to Purchase Mortgage-related Securities [Member]
Asset Derivatives [Abstract]
Notional Amount	34,545,000,000		32,707,000,000
Estimated Fair Value	103,000,000		309,000,000
Derivative Liabilities [Abstract]
Notional Amount	12,557,000,000		2,570,000,000
Estimated Fair Value	(23,000,000)		(6,000,000)
Mortgage Commitment Derivatives [Member] | Forward Contracts to Sell Mortgage-related Securities [Member]
Asset Derivatives [Abstract]
Notional Amount	18,886,000,000		1,370,000,000
Estimated Fair Value	26,000,000		3,000,000
Derivative Liabilities [Abstract]
Notional Amount	75,477,000,000		54,656,000,000
Estimated Fair Value	$ (266,000,000)		$ (548,000,000)

[1] The netting adjustment represents the effect of the legal right to offset under legally enforceable master netting agreements to settle with the same counterparty on a net basis, including cash collateral posted and received. Cash collateral posted was $6.3 billion and $6.8 billion as of December 31, 2012 and 2011, respectively. No cash collateral was received as of December 31, 2012 and $779 million was received as of December 31, 2011.

[2] Includes interest rate caps, futures, swap credit enhancements and mortgage insurance contracts that we account for as derivatives. The mortgage insurance contracts have payment provisions that are not based on a notional amount.