Investments in Securities Available-for-sale Securities 2 (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	$ 18,141	$ 27,413
Gross Unrealized Gains	2,170	3,448
Gross unrealized losses - OTTI	(39)	(55)
Gross unrealized losses, other	(42)	(152)
Available-for-sale securities	20,230	30,654
Fannie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	4,008	5,330
Gross Unrealized Gains	243	328
Gross unrealized losses - OTTI	0	0
Gross unrealized losses, other	(30)	(19)
Available-for-sale securities	4,221	5,639
Freddie Mac [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	4,000	5,100
Gross Unrealized Gains	299	428
Gross unrealized losses - OTTI	0	0
Gross unrealized losses, other	0	0
Available-for-sale securities	4,299	5,528
Ginnie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	343	416
Gross Unrealized Gains	48	60
Gross unrealized losses - OTTI	0	0
Gross unrealized losses, other	0	0
Available-for-sale securities	391	476
Alt-A private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,029	4,638
Gross Unrealized Gains	653	1,055
Gross unrealized losses - OTTI	(4)	(15)
Gross unrealized losses, other	0	0
Available-for-sale securities	2,678	5,678
Subprime private-label securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,526	4,103
Gross Unrealized Gains	759	1,161
Gross unrealized losses - OTTI	0	(9)
Gross unrealized losses, other	(4)	(15)
Available-for-sale securities	3,281	5,240
CMBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,235	1,341
Gross Unrealized Gains	20	56
Gross unrealized losses - OTTI	0	0
Gross unrealized losses, other	0	0
Available-for-sale securities	1,255	1,397
Mortgage revenue bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	2,639	3,859
Gross Unrealized Gains	99	177
Gross unrealized losses - OTTI	(29)	(8)
Gross unrealized losses, other	(8)	(5)
Available-for-sale securities	2,701	4,023
Other mortgage-related securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total amortized cost	1,361	2,626
Gross Unrealized Gains	49	183
Gross unrealized losses - OTTI	(6)	(23)
Gross unrealized losses, other	0	(113)
Available-for-sale securities	$ 1,404	$ 2,673