Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 3,508	$ 4,058
Gross unrealized gains	428	816
Gross unrealized losses	(48)	(31)
Total fair value	3,888	4,843
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,940	2,044
Gross unrealized gains	83	102
Gross unrealized losses	(37)	(27)
Total fair value	1,986	2,119
Other agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	305	332
Gross unrealized gains	22	25
Gross unrealized losses	0	0
Total fair value	327	357
Alt-A and subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	376	662
Gross unrealized gains	295	652
Gross unrealized losses	0	0
Total fair value	671	1,314
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	5	15
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total fair value	5	15
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	536	655
Gross unrealized gains	14	20
Gross unrealized losses	(11)	(4)
Total fair value	539	671
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	346	350
Gross unrealized gains	14	17
Gross unrealized losses	0	0
Total fair value	$ 360	$ 367