Reinsurance - Quota Share Transactions Terms (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2024	Sep. 30, 2024
2020 QSR and 2021 QSR - 2021 NIW
Effects of Reinsurance [Line Items]
Quota Share %		17.50%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		61.90%
2021 QSR and 2022 QSR - 2021 NIW
Effects of Reinsurance [Line Items]
Quota Share %		12.50%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		57.50%
2021 QSR and 2022 QSR - 2022 NIW
Effects of Reinsurance [Line Items]
Quota Share %		15.00%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		57.50%
Cede rate, option 1 (as a percent)		12.50%
Cede rate, option 2 (as a percent)		10.00%
2022 QSR and 2023 QSR - 2022 NIW
Effects of Reinsurance [Line Items]
Quota Share %		15.00%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		62.00%
Cede rate, option 1 (as a percent)		12.50%
Cede rate, option 2 (as a percent)		10.00%
2022 QSR and 2023 QSR - 2023 NIW
Effects of Reinsurance [Line Items]
Quota Share %		15.00%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		62.00%
Cede rate, option 1 (as a percent)		12.50%
Cede rate, option 2 (as a percent)		10.00%
2023 QSR - 2023 NIW
Effects of Reinsurance [Line Items]
Quota Share %		10.00%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		58.50%
Cede rate, option 1 (as a percent)		8.00%
Cede rate, option 2 (as a percent)		7.00%
2024 QSR - 2024 NIW
Effects of Reinsurance [Line Items]
Quota Share %		30.00%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		56.00%
Cede rate, option 1 (as a percent)		23.00%
Cede rate, option 2 (as a percent)		15.00%
Credit Union QSR Transaction
Effects of Reinsurance [Line Items]
Quota Share %		65.00%
Annual Loss Ratio to Exhaust Profit Commission (as a percent)		50.00%
Forecast | Quota Share Reinsurance Transaction, 2021
Effects of Reinsurance [Line Items]
Contingent termination fee	$ 8