Loss Reserves - Aging of Primary Default Inventory and Number of Delinquent Payments (Details) - loan	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Aging of the Primary Default Inventory [Abstract]
3 months or less	9,447	9,829	17,119
4 - 11 months	9,664	9,655	12,050
12 months or more	10,917	13,414	17,387
Total primary default inventory	30,028	32,898	46,556	50,282
3 months or less (in hundredths)	32.00%	30.00%	37.00%
4 - 11 months (in hundredths)	32.00%	29.00%	26.00%
12 months or more (in hundredths)	36.00%	41.00%	37.00%
Total primary default inventory (in hundredths)	100.00%	100.00%	100.00%
Primary claims received inventory included in ending default inventory	538	809	954