Loss Reserves - Aging of Primary Default Inventory (Details) - loan	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Aging of the Primary Default Inventory [Abstract]
3 months or less	11,542	9,447	9,829
4 - 11 months	34,620	9,664	9,655
12 months or more	11,548	10,917	13,414
Total primary default inventory	57,710	30,028	32,898	46,556
3 months or less (in hundredths)	20.00%	32.00%	30.00%
4 - 11 months (in hundredths)	60.00%	32.00%	29.00%
12 months or more (in hundredths)	20.00%	36.00%	41.00%
Total primary default inventory (in hundredths)	100.00%	100.00%	100.00%
Primary claims received inventory included in ending default inventory	159	538	809
Percent of inventory in default for more than 36 consecutive months	31.00%	36.00%	38.00%