Loss Reserves - Delinquent Items (Details) - loan	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Aging of the Primary Default Inventory [Abstract]
3 months or less (in loans)	10,299		12,194	11,547
3 months or less (as a percent)	25.00%		24.00%	22.00%
4 - 11 months (in loans)	11,018		13,450	12,680
4 - 11 months (as a percent)	27.00%		27.00%	24.00%
12 months or more (in loans)	20,000		24,638	28,331
12 months or more (as a percent)	48.00%		49.00%	54.00%
Total primary default inventory (in loans)	41,317	45,349	50,282	52,558	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)	1,258		1,385	1,829
Primary claims received inventory included in ending default inventory (as a percent)	3.00%		3.00%	3.00%
Percent of default inventory in default for more than 36 months (as a percent)	46.00%		47.00%	49.00%
Number of payments delinquent [Abstract]
3 payments or less (in loans)	15,858		18,419	17,299
3 payments or less (as a percent)	38.00%		36.00%	33.00%
4 - 11 payments (in loans)	10,560		12,892	12,746
4 - 11 payments (as a percent)	26.00%		26.00%	24.00%
12 payments or more (in loans)	14,899		18,971	22,513
12 payments or more (as a percent)	36.00%		38.00%	43.00%
Total primary default inventory (in loans)	41,317	45,349	50,282	52,558	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)	1,511		1,883	2,024