Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 3,338	$ 4,058
Gross unrealized gains	423	816
Gross unrealized losses	(38)	(31)
Total fair value	3,723	4,843
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,866	2,044
Gross unrealized gains	72	102
Gross unrealized losses	(34)	(27)
Total fair value	1,904	2,119
Other agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	279	332
Gross unrealized gains	19	25
Gross unrealized losses	0	0
Total fair value	298	357
Alt-A and subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	357	662
Gross unrealized gains	292	652
Gross unrealized losses	0	0
Total fair value	649	1,314
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost		15
Gross unrealized gains		0
Gross unrealized losses		0
Total fair value		15
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	494	655
Gross unrealized gains	16	20
Gross unrealized losses	(4)	(4)
Total fair value	506	671
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	342	350
Gross unrealized gains	24	17
Gross unrealized losses	0	0
Total fair value	$ 366	$ 367