Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 15,199	$ 18,141
Gross unrealized gains	1,834	2,170
Gross unrealized losses	(50)	(81)
Available-for-sale, at fair value	16,983	20,230
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	3,719	4,008
Gross unrealized gains	224	243
Gross unrealized losses	(30)	(30)
Available-for-sale, at fair value	3,913	4,221
Freddie Mac [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	3,449	4,000
Gross unrealized gains	252	299
Gross unrealized losses	0	0
Available-for-sale, at fair value	3,701	4,299
Ginnie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	221	343
Gross unrealized gains	33	48
Gross unrealized losses	0	0
Available-for-sale, at fair value	254	391
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,356	2,029
Gross unrealized gains	569	653
Gross unrealized losses	(5)	(4)
Available-for-sale, at fair value	1,920	2,678
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,929	2,526
Gross unrealized gains	620	759
Gross unrealized losses	(5)	(4)
Available-for-sale, at fair value	2,544	3,281
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,131	1,235
Gross unrealized gains	17	20
Gross unrealized losses	(1)	0
Available-for-sale, at fair value	1,147	1,255
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,473	2,639
Gross unrealized gains	100	99
Gross unrealized losses	(9)	(37)
Available-for-sale, at fair value	2,564	2,701
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	921	1,361
Gross unrealized gains	19	49
Gross unrealized losses	0	(6)
Available-for-sale, at fair value	$ 940	$ 1,404