Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 11,511	$ 18,141
Gross unrealized gains	1,705	2,170
Gross unrealized losses	(36)	(81)
Available-for-sale, at fair value	13,180	20,230
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	3,163	4,008
Gross unrealized gains	194	243
Gross unrealized losses	(21)	(30)
Available-for-sale, at fair value	3,336	4,221
Freddie Mac [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,899	4,000
Gross unrealized gains	139	299
Gross unrealized losses	0	0
Available-for-sale, at fair value	2,038	4,299
Ginnie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	58	343
Gross unrealized gains	8	48
Gross unrealized losses	0	0
Available-for-sale, at fair value	66	391
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,269	2,029
Gross unrealized gains	564	653
Gross unrealized losses	(6)	(4)
Available-for-sale, at fair value	1,827	2,678
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,722	2,526
Gross unrealized gains	656	759
Gross unrealized losses	(5)	(4)
Available-for-sale, at fair value	2,373	3,281
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,010	1,235
Gross unrealized gains	14	20
Gross unrealized losses	(1)	0
Available-for-sale, at fair value	1,023	1,255
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,919	2,639
Gross unrealized gains	113	99
Gross unrealized losses	(3)	(37)
Available-for-sale, at fair value	2,029	2,701
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	471	1,361
Gross unrealized gains	17	49
Gross unrealized losses	0	(6)
Available-for-sale, at fair value	$ 488	$ 1,404