Consolidated Statements Of Comprehensive Income (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:
Interest income	$ 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
Net interest income	423	381	384	408	262	233	200	129	1,597	824	177
Other income, net:
Gain on sale of agency securities, net	353	210	417	216	112	263	94	4	1,196	473	92
(Loss) gain on derivative instruments and other securities, net	89	(460)	(1,029)	47	(137)	(222)	(100)	12	(1,353)	(447)	38
Total other income, net	442	(250)	(612)	263	(25)	41	(6)	16	(157)	26	130
Expenses:
Management fees	31	32	28	22	18	16	12	8	113	55	11
General and administrative expenses	9	8	8	6	6	6	5	3	31	19	8
Total expenses	40	40	36	28	24	22	17	11	144	74	19
Income before income tax	825	91	(264)	643	213	252	177	134	1,296	776	288
Income tax benefit, net	15	5	(3)	2	5	1	0	0	19	6	0
Net income	810	86	(261)	641	208	251	177	134	1,277	770	288
Dividend on preferred stock	3	3	3	0					10	0	0
Comprehensive income available to common shareholders	123	1,324	477	587					2,511	1,149	200
Net (loss) income (attributable) available to common shareholders	807	83	(264)	641					1,267	770	288
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities, net		1,190	689	(106)	216	535	319	(41)	1,039	1,029	(64)
Unrealized (loss) gain on derivative instruments, net		51	52	52	54	(513)	(252)	61	205	(650)	(24)
Other comprehensive income (loss)	(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
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
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
Dividends declared per common share	$ 1.25	$ 1.25	$ 1.25	$ 1.25	$ 1.40	$ 1.40	$ 1.40	$ 1.40	$ 5.00	$ 5.60	$ 5.60