Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 2,539	$ 3,079
Gross unrealized gains	236	380
Gross unrealized losses	(14)	(30)
Total fair value	2,761	3,429
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,536	1,754
Gross unrealized gains	90	69
Gross unrealized losses	(11)	(26)
Total fair value	1,615	1,797
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	209	239
Gross unrealized gains	19	17
Gross unrealized losses	0	0
Total fair value	228	256
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	109	325
Gross unrealized gains	111	267
Gross unrealized losses	0	0
Total fair value	220	592
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	356	425
Gross unrealized gains	11	13
Gross unrealized losses	(3)	(4)
Total fair value	364	434
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	329	336
Gross unrealized gains	5	14
Gross unrealized losses	0	0
Total fair value	$ 334	$ 350