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Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Jan. 31, 2020 | Mar. 31, 2021 | Jun. 30, 2018 | Dec. 31, 2021 | Jan. 01, 2025 | Dec. 31, 2022 | Jun. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2012
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of risk premium ceded | 60.00% |  |  |  |  | 60.00% |  |  |  |  |
Insurance-linked securities, term |  |  |  | 12 years | 12 years |  |  |  |  |  |
Insurance-linked securities, callable term |  |  |  | 5 years | 5 years |  |  |  |  |  |
Cash and investments included in collateral trust | $ 481.7 |  |  |  |  | $ 481.7 |  |  |  |  |
HG Re |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00% |  |  |  |  | 15.00% |  |  |  |  |
Trust balance | $ 250.2 | $ 222.8 |  |  |  | $ 250.2 |  |  |  |  |
HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  | $ 503.0
Percentage of par value of policy reinsured | 15.00% |  |  |  |  | 15.00% |  |  |  |  |
Percent over a target trust balance |  |  |  |  |  | 102.00% |  |  |  |  |
Supplement trust target | $ 603.0 |  |  |  |  | $ 603.0 |  |  |  |  |
BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Principal | 89,196.5 | 75,287.7 |  | $ 800.0 |  | 89,196.5 |  |  |  | $ 1,100.0 |
Additional municipal bond guarantee |  |  |  |  |  |  |  |  | $ 36.9 |  |
HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Collateral held in supplement trust | 852.0 | 827.1 |  |  |  | 852.0 |  |  |  |  |
Cash and investments included in collateral trust | 481.7 | 434.5 |  |  |  | 481.7 |  |  |  |  |
Interest receivable | 157.6 | 155.7 |  |  |  | 157.6 |  |  |  |  |
BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Surplus notes | 364.6 | 388.2 |  |  |  | 364.6 |  |  |  |  |
Interest receivable | 5.7 | 4.4 |  |  |  | 5.7 |  |  |  |  |
Accrued interest on surplus notes | 157.6 | 155.7 |  |  |  | 157.6 |  |  |  |  |
Supplemental Trust | BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash repayments on notes | 23.6 | 21.5 | $ 47.9 |  |  |  |  |  |  |  |
Accrued interest on surplus notes | 0.4 | 0.2 | 0.9 |  |  | 0.4 |  |  |  |  |
Surplus Note | BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash repayments on notes | 33.8 | 30.1 | 65.0 |  |  |  |  |  |  |  |
Accrued interest on surplus notes | 9.8 | 8.4 | 16.2 |  |  | $ 9.8 |  |  |  |  |
BAM | Fidus Re Ltd. |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Losses above retention, percentage of liability |  |  |  | 90.00% | 90.00% |  |  |  |  |  |
Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Insurance linked securities |  |  |  | $ 150.0 | $ 100.0 |  |  |  |  |  |
Aggregate loss limit on contracts |  |  |  | 135.0 | 165.0 |  |  |  |  |  |
Reimbursement guarantee under contracts |  |  |  | $ 150.0 | 100.0 |  |  |  |  |  |
Percentage of financial guaranty policies covered |  |  |  |  |  | 38.00% |  |  |  |  |
Percentage of quota share reinsurance agreement |  |  |  | 100.00% |  |  |  |  |  |  |
Fidus Re Ltd. | BAM | 2018 Covered Portfolio |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of financial guaranty policies covered |  |  |  |  |  | 33.00% |  |  |  |  |
Maximum | BAM | HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Aggregate loss limit on contracts |  |  | $ 75.0 |  |  |  |  |  |  |  |
Maximum | Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Aggregate loss limit on contracts |  |  |  | $ 301.7 | $ 276.1 |  |  |  |  |  |
Series A BAM Surplus Note | HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Trust balance | $ 601.8 | $ 604.3 |  |  |  | $ 601.8 |  |  |  |  |
Fixed interest rate | Surplus Note | BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate | 8.00% |  |  |  |  | 8.00% |  |  |  |  |
Fixed interest rate | Surplus Note | BAM | Forecast |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  |  |  | 8.00% |  |  |  |
U.S. treasury rate plus | Surplus Note | BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  |  | 3.00% |  |  |  |  |
Effective yield (as a percent) | 3.20% |  |  |  |  | 3.20% |  |  |  |  |
U.S. treasury rate plus | Surplus Note | BAM | Forecast |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Effective yield (as a percent) |  |  |  |  |  |  |  | 3.20% |  |  |

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