Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 8,512	$ 18,141
Gross unrealized gains	1,383	2,170
Gross unrealized losses	(30)	(81)
Available-for-sale, at fair value	9,865	20,230
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,545	4,008
Gross unrealized gains	187	243
Gross unrealized losses	(20)	(30)
Available-for-sale, at fair value	2,712	4,221
Freddie Mac [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	534	4,000
Gross unrealized gains	48	299
Gross unrealized losses	0	0
Available-for-sale, at fair value	582	4,299
Ginnie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	54	343
Gross unrealized gains	7	48
Gross unrealized losses	0	0
Available-for-sale, at fair value	61	391
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	883	2,029
Gross unrealized gains	535	653
Gross unrealized losses	(1)	(4)
Available-for-sale, at fair value	1,417	2,678
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,437	2,526
Gross unrealized gains	486	759
Gross unrealized losses	(4)	(4)
Available-for-sale, at fair value	1,919	3,281
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	971	1,235
Gross unrealized gains	8	20
Gross unrealized losses	(1)	0
Available-for-sale, at fair value	978	1,255
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,639	2,639
Gross unrealized gains	90	99
Gross unrealized losses	(4)	(37)
Available-for-sale, at fair value	1,725	2,701
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	449	1,361
Gross unrealized gains	22	49
Gross unrealized losses	0	(6)
Available-for-sale, at fair value	$ 471	$ 1,404