Reinsurance - Excess of Loss Reinsurance (Details) - USD ($) $ in Thousands						9 Months Ended	12 Months Ended
	Aug. 03, 2021	Feb. 02, 2021	Oct. 29, 2020	May 25, 2019	Oct. 30, 2018	Sep. 30, 2021	Dec. 31, 2020
Home Re 2021-2
Effects of Reinsurance [Line Items]
Term of mortgage insurance-linked notes	12 years 6 months
Remaining First Layer Retention	$ 190,159					$ 190,159
Remaining Excess of Loss Reinsurance Coverage	$ 398,429					398,429
Home Re 2021-1
Effects of Reinsurance [Line Items]
Term of mortgage insurance-linked notes		12 years 6 months
Remaining First Layer Retention		$ 211,159				211,159
Remaining Excess of Loss Reinsurance Coverage		$ 398,848				398,848
Home Re 2020-1
Effects of Reinsurance [Line Items]
Term of mortgage insurance-linked notes			10 years
Remaining First Layer Retention			$ 275,283			275,268	$ 275,283
Remaining Excess of Loss Reinsurance Coverage			$ 412,917			288,777	412,917
Home Re 2019-1
Effects of Reinsurance [Line Items]
Term of mortgage insurance-linked notes				10 years
Remaining First Layer Retention				$ 185,730		184,007	184,514
Remaining Excess of Loss Reinsurance Coverage				$ 315,739		208,146	208,146
Home Re 2018-1
Effects of Reinsurance [Line Items]
Term of mortgage insurance-linked notes					10 years
Remaining First Layer Retention					$ 168,691	165,434	166,005
Remaining Excess of Loss Reinsurance Coverage					$ 318,636	$ 218,343	$ 218,343