Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 4,058	$ 7,261
Gross unrealized gains	816	1,142
Gross unrealized losses	(31)	(40)
Total fair value	4,843	8,363
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,044	2,445
Gross unrealized gains	102	137
Gross unrealized losses	(27)	(28)
Total fair value	2,119	2,554
Other agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	332	508
Gross unrealized gains	25	39
Gross unrealized losses	0	0
Total fair value	357	547
Alt-A and subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	662	1,817
Gross unrealized gains	652	895
Gross unrealized losses	0	(3)
Total fair value	1,314	2,709
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	15	815
Gross unrealized gains	0	4
Gross unrealized losses	0	0
Total fair value	15	819
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	655	1,245
Gross unrealized gains	20	36
Gross unrealized losses	(4)	(9)
Total fair value	671	1,272
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	350	431
Gross unrealized gains	17	31
Gross unrealized losses	0	0
Total fair value	$ 367	$ 462