Loss Reserves - Aging of Primary Default Inventory and Number of Delinquent Payments (Details) - loan	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Aging of the Primary Default Inventory [Abstract]
3 months or less	9,829	17,119	12,194
4 - 11 months	9,655	12,050	13,450
12 months or more	13,414	17,387	24,638
Total primary default inventory	32,898	46,556	50,282	62,633
3 months or less (in hundredths)	30.00%	37.00%	24.00%
4 - 11 months (in hundredths)	29.00%	26.00%	27.00%
12 months or more (in hundredths)	41.00%	37.00%	49.00%
Total primary default inventory (in hundredths)	100.00%	100.00%	100.00%
Primary claims received inventory included in ending default inventory	809	954	1,385