Loss Reserves - Delinquent Items (Details) - loan	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Aging of the Primary Delinquent Inventory [Abstract]
3 months or less (in loans)	8,770	17,119	9,184
3 months or less (as a percent)	21.00%	37.00%	20.00%
4 - 11 months (in loans)	16,429	12,050	13,617
4 - 11 months (as a percent)	40.00%	26.00%	30.00%
12 months or more (in loans)	16,044	17,387	22,548
12 months or more (as a percent)	39.00%	37.00%	50.00%
Total primary delinquent inventory (in loans)	41,243	46,556	45,349	50,282
Total primary delinquent inventory (as a percent)	100.00%	100.00%	100.00%
Primary claims received inventory included in ending delinquent inventory (in loans)	819	954	1,390
Primary claims received inventory included in ending delinquent inventory (as a percent)	2.00%	2.00%	3.00%
Percent of 12 months or more delinquent inventory, delinquent for more than 36 months (as a percent)	44.00%	45.00%	48.00%
Number of payments delinquent [Abstract]
3 payments or less (in loans)	16,023	21,678	15,692
3 payments or less (as a percent)	39.00%	46.00%	35.00%
4 - 11 payments (in loans)	13,734	12,446	12,275
4 - 11 payments (as a percent)	33.00%	27.00%	27.00%
12 payments or more (in loans)	11,486	12,432	17,382
12 payments or more (as a percent)	28.00%	27.00%	38.00%
Total primary delinquent inventory (in loans)	41,243	46,556	45,349	50,282
Total primary delinquent inventory (as a percent)	100.00%	100.00%	100.00%
Percent of 12 payments or more delinquent inventory, 36 payments or more delinquent (as a percent)	42.00%	43.00%	45.00%
Pool insurance delinquent inventory (in loans)	1,200	1,309	1,714