Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 5,419	$ 7,261
Gross unrealized gains	1,021	1,142
Gross unrealized losses	(32)	(40)
Available-for-sale, at fair value	6,408	8,363
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,173	2,445
Gross unrealized gains	124	137
Gross unrealized losses	(25)	(28)
Available-for-sale, at fair value	2,272	2,554
Other agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	399	508
Gross unrealized gains	33	39
Gross unrealized losses	0	0
Available-for-sale, at fair value	432	547
Alt-A and subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,335	1,817
Gross unrealized gains	828	895
Gross unrealized losses	(1)	(3)
Available-for-sale, at fair value	2,162	2,709
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	259	815
Gross unrealized gains	0	4
Gross unrealized losses	0	0
Available-for-sale, at fair value	259	819
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	858	1,245
Gross unrealized gains	21	36
Gross unrealized losses	(6)	(9)
Available-for-sale, at fair value	873	1,272
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	395	431
Gross unrealized gains	15	31
Gross unrealized losses	0	0
Available-for-sale, at fair value	$ 410	$ 462