Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 7,261	$ 18,141
Gross unrealized gains	1,142	2,170
Gross unrealized losses	(40)	(81)
Total fair value	8,363	20,230
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,445	4,008
Gross unrealized gains	137	243
Gross unrealized losses	(28)	(30)
Total fair value	2,554	4,221
Freddie Mac [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	474	4,000
Gross unrealized gains	35	299
Gross unrealized losses	0	0
Total fair value	509	4,299
Ginnie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	34	343
Gross unrealized gains	4	48
Gross unrealized losses	0	0
Total fair value	38	391
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	747	2,029
Gross unrealized gains	544	653
Gross unrealized losses	0	(4)
Total fair value	1,291	2,678
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,070	2,526
Gross unrealized gains	351	759
Gross unrealized losses	(3)	(4)
Total fair value	1,418	3,281
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	815	1,235
Gross unrealized gains	4	20
Gross unrealized losses	0	0
Total fair value	819	1,255
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,245	2,639
Gross unrealized gains	36	99
Gross unrealized losses	(9)	(37)
Total fair value	1,272	2,701
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	431	1,361
Gross unrealized gains	31	49
Gross unrealized losses	0	(6)
Total fair value	$ 462	$ 1,404