Loss Reserves - Delinquent Items (Details) - loan	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Aging of the Primary Delinquent Inventory [Abstract]
3 months or less (in loans)	8,554		17,119	10,299
3 months or less (as a percent)	24.00%		37.00%	25.00%
4 - 11 months (in loans)	12,506		12,050	11,018
4 - 11 months (as a percent)	35.00%		26.00%	27.00%
12 months or more (in loans)	14,977		17,387	20,000
12 months or more (as a percent)	41.00%		37.00%	48.00%
Total primary delinquent inventory (in loans)	36,037	41,243	46,556	41,317	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)	827		954	1,258
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)	43.00%		45.00%	46.00%
Number of payments delinquent [Abstract]
3 payments or less (in loans)	14,178		21,678	15,858
3 payments or less (as a percent)	39.00%		46.00%	38.00%
4 - 11 payments (in loans)	11,429		12,446	10,560
4 - 11 payments (as a percent)	32.00%		27.00%	26.00%
12 payments or more (in loans)	10,430		12,432	14,899
12 payments or more (as a percent)	29.00%		27.00%	36.00%
Total primary delinquent inventory (in loans)	36,037	41,243	46,556	41,317	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)	41.00%		43.00%	44.00%
Pool insurance delinquent inventory (in loans)	1,067		1,309	1,511