Loss Reserves - Delinquent Items (Details) - loan	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Aging of the Primary Delinquent Inventory [Abstract]
3 months or less (in loans)	9,484		17,119	11,331
3 months or less (as a percent)	28.00%		37.00%	27.00%
4 - 11 months (in loans)	9,564		12,050	11,092
4 - 11 months (as a percent)	29.00%		26.00%	27.00%
12 months or more (in loans)	14,350		17,387	18,812
12 months or more (as a percent)	43.00%		37.00%	46.00%
Total primary delinquent inventory (in loans)	33,398	36,037	46,556	41,235	41,317	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)	766		954	1,063
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)	39.00%		45.00%	45.00%
Number of payments delinquent [Abstract]
3 payments or less (in loans)	14,813		21,678	16,916
3 payments or less (as a percent)	44.00%		46.00%	41.00%
4 - 11 payments (in loans)	9,156		12,446	10,583
4 - 11 payments (as a percent)	28.00%		27.00%	26.00%
12 payments or more (in loans)	9,429		12,432	13,736
12 payments or more (as a percent)	28.00%		27.00%	33.00%
Total primary delinquent inventory (in loans)	33,398	36,037	46,556	41,235	41,317	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)	39.00%		43.00%	43.00%
Pool insurance delinquent inventory (in loans)	1,032		1,309	1,426