Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 4,932	$ 7,261
Gross unrealized gains	1,063	1,142
Gross unrealized losses	(32)	(40)
Available-for-sale, at fair value	5,963	8,363
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,118	2,445
Gross unrealized gains	116	137
Gross unrealized losses	(26)	(28)
Available-for-sale, at fair value	2,208	2,554
Other agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	361	508
Gross unrealized gains	30	39
Gross unrealized losses	0	0
Available-for-sale, at fair value	391	547
Alt-A and subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,155	1,817
Gross unrealized gains	879	895
Gross unrealized losses	0	(3)
Available-for-sale, at fair value	2,034	2,709
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	182	815
Gross unrealized gains	0	4
Gross unrealized losses	0	0
Available-for-sale, at fair value	182	819
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	736	1,245
Gross unrealized gains	19	36
Gross unrealized losses	(6)	(9)
Available-for-sale, at fair value	749	1,272
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	380	431
Gross unrealized gains	19	31
Gross unrealized losses	0	0
Available-for-sale, at fair value	$ 399	$ 462