Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	$ 2,910	$ 3,079
Gross unrealized gains	327	380
Gross unrealized losses	(23)	(30)
Total fair value	3,214	3,429
Fannie Mae [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	1,689	1,754
Gross unrealized gains	82	69
Gross unrealized losses	(19)	(26)
Total fair value	1,752	1,797
Other agency [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	225	239
Gross unrealized gains	17	17
Gross unrealized losses	0	0
Total fair value	242	256
Alt-A and subprime private-label securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	246	325
Gross unrealized gains	206	267
Gross unrealized losses	0	0
Total fair value	452	592
Mortgage revenue bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	416	425
Gross unrealized gains	13	13
Gross unrealized losses	(4)	(4)
Total fair value	425	434
Other mortgage-related securities [Member]
Debt Securities, Available-for-sale [Line Items]
Total amortized cost	334	336
Gross unrealized gains	9	14
Gross unrealized losses	0	0
Total fair value	$ 343	$ 350