Derivative Instruments Notional and FV Position (Details) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Asset Derivatives [Abstract]
Notional Amount	$ 445,524,000,000		$ 336,877,000,000
Estimated Fair Value	1,328,000,000		561,000,000
Derivative Liabilities [Abstract]
Notional Amount	418,700,000,000		377,748,000,000
Estimated Fair Value	(2,033,000,000)		(916,000,000)
Table Footnote [Abstract]
Cash collateral posted	7,800,000,000		6,800,000,000
Cash collateral received	0		779,000,000
Risk Management Derivatives [Member]
Asset Derivatives [Abstract]
Notional Amount	374,638,000,000		293,090,000,000
Estimated Fair Value	15,859,000,000		15,085,000,000
Accrued interest receivable	1,224,000,000		920,000,000
Netting adjustment	(16,826,000,000)	[1]	(15,829,000,000)	[1]
Total net risk management derivatives	257,000,000		176,000,000
Derivative Liabilities [Abstract]
Notional Amount	310,045,000,000		320,100,000,000
Estimated Fair Value	(23,369,000,000)		(21,022,000,000)
Accrued interest payable	(1,602,000,000)		(1,238,000,000)
Netting adjustment	24,596,000,000	[1]	21,898,000,000	[1]
Total net risk management derivatives	(375,000,000)		(362,000,000)
Risk Management Derivatives [Member] | Pay-fixed Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	5,375,000,000		30,950,000,000
Estimated Fair Value	80,000,000		102,000,000
Derivative Liabilities [Abstract]
Notional Amount	244,217,000,000		155,807,000,000
Estimated Fair Value	(20,619,000,000)		(17,391,000,000)
Risk Management Derivatives [Member] | Receive-fixed Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	286,365,000,000		170,668,000,000
Estimated Fair Value	10,971,000,000		8,118,000,000
Derivative Liabilities [Abstract]
Notional Amount	9,075,000,000		59,027,000,000
Estimated Fair Value	(27,000,000)		(93,000,000)
Risk Management Derivatives [Member] | Basis Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	21,822,000,000		382,000,000
Estimated Fair Value	163,000,000		122,000,000
Derivative Liabilities [Abstract]
Notional Amount	3,100,000,000		9,240,000,000
Estimated Fair Value	(1,000,000)		(44,000,000)
Risk Management Derivatives [Member] | Foreign Currency Swap [Member]
Asset Derivatives [Abstract]
Notional Amount	811,000,000		581,000,000
Estimated Fair Value	195,000,000		155,000,000
Derivative Liabilities [Abstract]
Notional Amount	391,000,000		451,000,000
Estimated Fair Value	(43,000,000)		(62,000,000)
Risk Management Derivatives [Member] | Pay-fixed Swaption [Member]
Asset Derivatives [Abstract]
Notional Amount	33,400,000,000		48,600,000,000
Estimated Fair Value	105,000,000		165,000,000
Derivative Liabilities [Abstract]
Notional Amount	26,625,000,000		47,750,000,000
Estimated Fair Value	(162,000,000)		(194,000,000)
Risk Management Derivatives [Member] | Receive-fixed Swaption [Member]
Asset Derivatives [Abstract]
Notional Amount	19,470,000,000		33,695,000,000
Estimated Fair Value	4,317,000,000		6,371,000,000
Derivative Liabilities [Abstract]
Notional Amount	26,625,000,000		47,750,000,000
Estimated Fair Value	(2,517,000,000)		(3,238,000,000)
Risk Management Derivatives [Member] | Other [Member]
Asset Derivatives [Abstract]
Notional Amount	7,395,000,000	[2]	8,214,000,000	[2]
Estimated Fair Value	28,000,000	[2]	52,000,000	[2]
Derivative Liabilities [Abstract]
Notional Amount	12,000,000	[2]	75,000,000	[2]
Estimated Fair Value	0	[2]	0	[2]
Mortgage Commitment Derivatives [Member]
Asset Derivatives [Abstract]
Notional Amount	70,886,000,000		43,787,000,000
Estimated Fair Value	1,071,000,000		385,000,000
Derivative Liabilities [Abstract]
Notional Amount	108,655,000,000		57,648,000,000
Estimated Fair Value	(1,658,000,000)		(554,000,000)
Mortgage Commitment Derivatives [Member] | Mortgage Commitments to Purchase Whole Loans [Member]
Asset Derivatives [Abstract]
Notional Amount	16,260,000,000		9,710,000,000
Estimated Fair Value	255,000,000		73,000,000
Derivative Liabilities [Abstract]
Notional Amount	4,209,000,000		422,000,000
Estimated Fair Value	(6,000,000)		0
Mortgage Commitment Derivatives [Member] | Forward Contracts to Purchase Mortgage-related Securities [Member]
Asset Derivatives [Abstract]
Notional Amount	48,212,000,000		32,707,000,000
Estimated Fair Value	804,000,000		309,000,000
Derivative Liabilities [Abstract]
Notional Amount	7,447,000,000		2,570,000,000
Estimated Fair Value	(14,000,000)		(6,000,000)
Mortgage Commitment Derivatives [Member] | Forward Contracts to Sell Mortgage-related Securities [Member]
Asset Derivatives [Abstract]
Notional Amount	6,414,000,000		1,370,000,000
Estimated Fair Value	12,000,000		3,000,000
Derivative Liabilities [Abstract]
Notional Amount	96,999,000,000		54,656,000,000
Estimated Fair Value	$ (1,638,000,000)		$ (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 $7.8 billion and $6.8 billion as of September 30, 2012 and December 31, 2011, respectively. No cash collateral was received as of September 30, 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.