Loss Reserves - Delinquent Items (Details) $ in Millions	Mar. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan	Mar. 31, 2016 loan	Dec. 31, 2015 loan
Aging of the Primary Default Inventory [Abstract]
3 months or less (in loans)	9,184	12,194	10,120
3 months or less (as a percent)	20.00%	24.00%	18.00%
4 - 11 months (in loans)	13,617	13,450	15,319
4 - 11 months (as a percent)	30.00%	27.00%	28.00%
12 months or more (in loans)	22,548	24,638	30,151
12 months or more (as a percent)	50.00%	49.00%	54.00%
Total primary default inventory (in loans)	45,349	50,282	55,590	62,633
Total primary default inventory (as a percent)	100.00%	100.00%	100.00%
Primary claims received inventory included in ending default inventory (in loans)	1,390	1,385	2,267
Primary claims received inventory included in ending default inventory (as a percent)	3.00%	3.00%	4.00%
Percent of default inventory in default for more than 36 months (as a percent)	48.00%	47.00%	49.00%
Number of payments delinquent [Abstract]
3 payments or less (in loans)	15,692	18,419	16,864
3 payments or less (as a percent)	35.00%	36.00%	30.00%
4 - 11 payments (in loans)	12,275	12,892	14,595
4 - 11 payments (as a percent)	27.00%	26.00%	26.00%
12 payments or more (in loans)	17,382	18,971	24,131
12 payments or more (as a percent)	38.00%	38.00%	44.00%
Total primary default inventory (in loans)	45,349	50,282	55,590	62,633
Total primary default inventory (as a percent)	100.00%	100.00%	100.00%
Pool insurance notice inventory (in number of loans)	1,714	1,883	2,247
Premium refund liability, expected future rescissions | $	$ 5	$ 5