Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 2,448	$ 3,079
Gross unrealized gains	255	380
Gross unrealized losses	(13)	(30)
Total fair value	2,690	3,429
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,492	1,754
Gross unrealized gains	98	69
Gross unrealized losses	(8)	(26)
Total fair value	1,582	1,797
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	195	239
Gross unrealized gains	18	17
Gross unrealized losses	0	0
Total fair value	213	256
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	102	325
Gross unrealized gains	124	267
Gross unrealized losses	0	0
Total fair value	226	592
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	339	425
Gross unrealized gains	10	13
Gross unrealized losses	(3)	(4)
Total fair value	346	434
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	320	336
Gross unrealized gains	5	14
Gross unrealized losses	(2)	0
Total fair value	$ 323	$ 350