Quarterly Results (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Interest Income, Securities, Mortgage Backed	$ 570	$ 520	$ 504	$ 514	$ 353	$ 327	$ 265	$ 165	$ 2,109	$ 1,109	$ 253
Interest Expense	147	139	120	106	91	94	65	36	512	285	76
Gain (Loss) on Sales of Mortgage Backed Securities (MBS)	353	210	417	216	112	263	94	4	1,196	473	92
Unrealized Gain (Loss) On Derivative Instrument And Trading Securities	89	(460)	(1,029)	47	(137)	(222)	(100)	12	(1,353)	(447)	38
Management fees	31	32	28	22	18	16	12	8	113	55	11
General and Administrative Expense	9	8	8	6	6	6	5	3	31	19	8
Excise and REIT Income tax	15	5	(3)	2	5	1	0	0	19	6	0
Net income (loss)	810	86	(261)	641	208	251	177	134	1,277	770	288
Unrealized gain (loss) on available-for-sale securities, net		1,190	689	(106)	216	535	319	(41)	1,039	1,029	(64)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax		51	52	52	54	(513)	(252)	61	205	(650)	(24)
Preferred Stock Dividends, Income Statement Impact	3	3	3	0					10	0	0
Comprehensive income available to common shareholders	123	1,324	477	587					2,511	1,149	200
Net income (loss) available (attributable) to common shareholders	807	83	(264)	641					1,267	770	288
Weighted Average Number Of Common Shares Outstanding Basic And Diluted	340.3	332.8	301.0	240.6	210.3	180.7	130.5	90.3	303.9	153.3	36.5
Common Stock, Dividends, Per Share, Declared	$ 1.25	$ 1.25	$ 1.25	$ 1.25	$ 1.40	$ 1.40	$ 1.40	$ 1.40	$ 5.00	$ 5.60	$ 5.60
Net interest income	423	381	384	408	262	233	200	129	1,597	824	177
Total other income (loss), net	442	(250)	(612)	263	(25)	41	(6)	16	(157)	26	130
Total expenses	40	40	36	28	24	22	17	11	144	74	19
Income (loss) before income tax (benefit)	825	91	(264)	643	213	252	177	134	1,296	776	288
Other comprehensive income	(684)	1,241	741	(54)	270	22	67	20	1,244	379	(88)
Comprehensive income	126	1,327	480	587	478	273	244	154	2,521	1,149	200
Net income per common share - basic and diluted	$ 2.37	$ 0.25	$ (0.88)	$ 2.66	$ 0.99	$ 1.39	$ 1.36	$ 1.48	$ 4.17	$ 5.02	$ 7.89
Comprehensive income per share - basic and diluted	$ 0.36	$ 3.98	$ 1.58	$ 2.44	$ 2.27	$ 1.51	$ 1.87	$ 1.71	$ 8.26	$ 7.50	$ 5.49
Accumulated Other Comprehensive Income (Loss) [Member]
Unrealized gain (loss) on available-for-sale securities, net	(734)								1,039	1,029	(64)
Interest Rate Swap [Member]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax										(650)	(24)
Interest Rate Swap [Member] | Discontinuation of Election to Account for Interest Rate Swaps as Designated Cash Flow Hedges [Member]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	$ 50								$ 205	$ 54