Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 2,281	$ 3,079
Gross unrealized gains	137	380
Gross unrealized losses	(14)	(30)
Total fair value	2,404	3,429
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,445	1,754
Gross unrealized gains	85	69
Gross unrealized losses	(10)	(26)
Total fair value	1,520	1,797
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	183	239
Gross unrealized gains	15	17
Gross unrealized losses	0	0
Total fair value	198	256
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	34	325
Gross unrealized gains	23	267
Gross unrealized losses	0	0
Total fair value	57	592
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	309	425
Gross unrealized gains	9	13
Gross unrealized losses	(3)	(4)
Total fair value	315	434
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	310	336
Gross unrealized gains	5	14
Gross unrealized losses	(1)	0
Total fair value	$ 314	$ 350