Derivative Instruments Derivatives 1 - Notional and FV Position (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Asset Derivatives [Abstract]
Notional amount	$ 247,985,000,000	$ 183,204,000,000
Derivative asset, estimated fair value	1,692,000,000	1,485,000,000
Netting adjustment	(5,626,000,000)	(5,186,000,000)
Liability Derivatives [Abstract]
Notional amount	395,719,000,000	408,714,000,000
Derivative liability, estimated fair value	(802,000,000)	(614,000,000)
Netting adjustment	12,036,000,000	10,194,000,000
Table Footnote [Abstract]
Cash collateral posted for derivative instruments	6,600,000,000	5,300,000,000
Cash collateral received for derivative instruments	161,000,000	245,000,000
Risk Management Derivatives [Member]
Asset Derivatives [Abstract]
Notional amount	169,716,000,000	128,628,000,000
Derivative asset, estimated fair value	5,989,000,000	5,667,000,000
Accrued interest receivable, estimated fair value	917,000,000	749,000,000
Netting adjustment	(5,626,000,000)	(5,186,000,000)
Total net risk management derivatives	1,280,000,000	1,230,000,000
Liability Derivatives [Abstract]
Notional amount	295,920,000,000	344,264,000,000
Derivative liability, estimated fair value	(11,039,000,000)	(9,451,000,000)
Accrued interest payable, estimated fair value	(1,232,000,000)	(1,013,000,000)
Netting adjustment	12,036,000,000	10,194,000,000
Total net risk management derivatives	(235,000,000)	(270,000,000)
Risk Management Derivatives [Member] | Pay-fixed Swap [Member]
Asset Derivatives [Abstract]
Notional amount	17,739,000,000	41,965,000,000
Derivative asset, estimated fair value	114,000,000	733,000,000
Liability Derivatives [Abstract]
Notional amount	144,469,000,000	123,557,000,000
Derivative liability, estimated fair value	(9,424,000,000)	(7,125,000,000)
Risk Management Derivatives [Member] | Receive-fixed Swap [Member]
Asset Derivatives [Abstract]
Notional amount	141,195,000,000	67,629,000,000
Derivative asset, estimated fair value	5,508,000,000	4,486,000,000
Liability Derivatives [Abstract]
Notional amount	97,104,000,000	157,272,000,000
Derivative liability, estimated fair value	(516,000,000)	(1,302,000,000)
Risk Management Derivatives [Member] | Basis Swap [Member]
Asset Derivatives [Abstract]
Notional amount	1,769,000,000	5,769,000,000
Derivative asset, estimated fair value	156,000,000	123,000,000
Liability Derivatives [Abstract]
Notional amount	9,100,000,000	7,100,000,000
Derivative liability, estimated fair value	(3,000,000)	(2,000,000)
Risk Management Derivatives [Member] | Foreign Currency Swap [Member]
Asset Derivatives [Abstract]
Notional amount	328,000,000	344,000,000
Derivative asset, estimated fair value	132,000,000	144,000,000
Liability Derivatives [Abstract]
Notional amount	260,000,000	273,000,000
Derivative liability, estimated fair value	(48,000,000)	(30,000,000)
Risk Management Derivatives [Member] | Pay-fixed Swaption [Member]
Asset Derivatives [Abstract]
Notional amount	7,625,000,000	11,100,000,000
Derivative asset, estimated fair value	52,000,000	57,000,000
Liability Derivatives [Abstract]
Notional amount	21,700,000,000	26,525,000,000
Derivative liability, estimated fair value	(79,000,000)	(175,000,000)
Risk Management Derivatives [Member] | Receive-fixed Swaption [Member]
Asset Derivatives [Abstract]
Notional amount	0	750,000,000
Derivative asset, estimated fair value	0	96,000,000
Liability Derivatives [Abstract]
Notional amount	23,275,000,000	29,525,000,000
Derivative liability, estimated fair value	(968,000,000)	(816,000,000)
Risk Management Derivatives [Member] | Other [Member]
Asset Derivatives [Abstract]
Notional amount	1,060,000,000	1,071,000,000
Derivative asset, estimated fair value	27,000,000	28,000,000
Liability Derivatives [Abstract]
Notional amount	12,000,000	12,000,000
Derivative liability, estimated fair value	(1,000,000)	(1,000,000)
Mortgage commitment derivatives [Member]
Asset Derivatives [Abstract]
Notional amount	78,269,000,000	54,576,000,000
Derivative asset, estimated fair value	412,000,000	255,000,000
Netting adjustment	0	0
Liability Derivatives [Abstract]
Notional amount	99,799,000,000	64,450,000,000
Derivative liability, estimated fair value	(567,000,000)	(344,000,000)
Netting adjustment	0	0
Mortgage commitment derivatives [Member] | Mortgage Commitments to Purchase Whole Loans [Member]
Asset Derivatives [Abstract]
Notional amount	10,387,000,000	6,157,000,000
Derivative asset, estimated fair value	51,000,000	28,000,000
Liability Derivatives [Abstract]
Notional amount	1,493,000,000	428,000,000
Derivative liability, estimated fair value	(4,000,000)	0
Mortgage commitment derivatives [Member] | Forward Contracts to Purchase Mortgage-related Securities [Member]
Asset Derivatives [Abstract]
Notional amount	59,334,000,000	43,533,000,000
Derivative asset, estimated fair value	341,000,000	223,000,000
Liability Derivatives [Abstract]
Notional amount	6,748,000,000	6,112,000,000
Derivative liability, estimated fair value	(14,000,000)	(8,000,000)
Mortgage commitment derivatives [Member] | Forward Contracts to Sell Mortgage-related Securities [Member]
Asset Derivatives [Abstract]
Notional amount	8,548,000,000	4,886,000,000
Derivative asset, estimated fair value	20,000,000	4,000,000
Liability Derivatives [Abstract]
Notional amount	91,558,000,000	57,910,000,000
Derivative liability, estimated fair value	$ (549,000,000)	$ (336,000,000)