Quarterly Results (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest income	$ 386	$ 318	$ 293	$ 296	$ 393	$ 315	$ 318	$ 295	$ 1,293	$ 1,321	$ 1,466
Interest expense	174	140	112	98	98	96	101	99	524	394	330
Net interest income	212	178	181	198	295	219	217	196	769	927	1,136
Noninterest Income [Abstract]
Gain (loss) on sale of investment securities, net	(16)	22	15	(84)	(5)	61	55	(2)	(63)	109	(23)
Unrealized gain (loss) on investment securities measured at fair value through net income, net	(65)	(31)	9	16	(11)	(6)	0	11	(71)	(6)	5
(Loss) gain on derivative instruments and other securities, net	271	131	(169)	(40)	753	248	(367)	(944)	193	(310)	(764)
Management Fees Revenue	$ 3	$ 3	$ 4	$ 3	4	4	0	0	$ 13	$ 8	$ 0
Weighted Average Number of Shares Outstanding, Diluted	391.5	364.9	346.5	331.1					358.7	331.9	348.6
Total other (loss) income, net	$ 193	$ 125	$ (141)	$ (105)	741	307	(312)	(935)	$ 72	$ (199)	$ (782)
Management fees					0	0	25	27	0	52	116
Labor and Related Expense	12	10	10	10	10	9	0	0	42	19	0
General and administrative expenses	8	7	6	7	7	6	15	6	28	34	23
Total expenses	20	17	16	17	17	15	40	33	70	105	139
Net income (loss)	385	286	24	76	1,019	511	(135)	(772)	771	623	215
Dividend on preferred stock	9	9	7	7	7	7	7	7	32	28	28
Net income (loss) available (attributable) to common shareholders	376	271	17	69	1,012	504	(142)	(779)	733	595	187
Unrealized Gains and (Losses), Net	(205)	90	121	46	(1,408)	(97)	370	765	52	(370)	(597)
Issuance costs of redeemed preferred stock	$ 0	$ 6	$ 0	$ 0					$ 6	$ 0	$ 0
Weighted Average Number of Shares Outstanding, Basic	391.3	364.7	346.4	331.0					358.6	331.9	348.6
Unrealized gain on derivative instruments, net					1	7	12	19	$ 0	$ 39	$ 101
Other comprehensive (loss) income					(1,407)	(90)	382	784	52	(331)	(496)
Comprehensive (loss) income	$ 180	$ 376	$ 145	$ 122	(388)	421	247	12	823	292	(281)
Comprehensive (loss) income (attributable) available to common shareholders	$ 171	$ 361	$ 138	$ 115	$ (395)	$ 414	$ 240	$ 5	$ 785	$ 264	$ (309)
Earnings Per Share, Basic and Diluted	$ 0.96	$ 0.74	$ 0.05	$ 0.21	$ 3.06	$ 1.52	$ (0.43)	$ (2.33)	$ 2.04	$ 1.79	$ 0.54
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Common Stock, Dividends, Per Share, Declared	0.54	0.54	0.54	0.54	$ 0.54	$ 0.56	$ 0.60	$ 0.60
Weighted Average Number of Shares Outstanding, Basic and Diluted					331.0	331.0	331.0	334.4
Comprehensive income per share - basic and diluted	$ 0.44	$ 0.99	$ 0.40	$ 0.35	$ (1.19)	$ 1.25	$ 0.73	$ 0.01