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Municipal Bond Guarantee Insurance - Narrative (Details) - USD ($) $ in Millions |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 17, 2021 | Jan. 31, 2021 | Dec. 31, 2020 | Jan. 31, 2020 | Nov. 30, 2018 | Apr. 30, 2018 | Mar. 31, 2021 | Dec. 31, 2012 | Dec. 31, 2021 | Sep. 30, 2019 | Dec. 31, 2017 | Jul. 31, 2012
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of risk premium ceded |  |  |  |  |  |  | 60.00% |  |  |  |  |
Insurance-linked securities, term |  |  |  |  |  | 12 years |  |  |  |  |  |
Insurance-linked securities, callable term |  |  |  |  |  | 5 years |  |  |  |  |  |
Percent change in discounted future cash flows |  |  |  |  |  |  | 10.00% |  |  |  |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Collateral held in supplement trust |  |  | $ 827.1 |  |  |  | $ 819.1 |  |  |  |  |
Collateral Trusts, Investments And Cash |  |  | 434.5 |  |  |  | 427.9 |  |  |  |  |
Interest receivable |  |  | 155.7 |  |  |  | 158.8 |  |  |  |  |
BAM |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  | 388.2 |  |  |  | 388.2 |  |  |  |  |
Interest receivable |  |  | $ 4.4 |  |  |  | $ 3.0 |  |  |  |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  |  |  |  |  |  | 15.00% |  |  |  |  |
Capital invested |  |  |  |  |  |  |  | $ 594.5 |  |  |  |
Surplus notes |  |  |  |  |  |  |  | 503.0 |  |  |  | $ 503.0
Cash and Fixed Income Securities |  |  |  |  |  |  |  |  |  |  |  | 100.0
Percent over a target trust balance |  |  |  |  |  |  | 102.00% |  |  |  |  |
Supplement Trust Target |  |  |  |  |  |  | $ 603.0 |  |  |  |  |
HG Global | Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 96.90% |  |  |  | 96.90% |  |  |  |  |
HG Global | Common equity securities |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  | 88.40% |  |  |  | 88.40% |  |  |  |  |
HG Global | Series B BAM Surplus Note |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  | 300.0
HG Global | Series A BAM Surplus Note |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  | $ 203.0 | $ 203.0
Restricted Cash |  |  | $ 604.3 |  |  |  | $ 597.9 |  |  |  |  |
HG Global | BAM | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Aggregate losses on ceded business |  |  |  | $ 75.0 |  |  |  |  |  |  |  |
HG Global | Non-controlling interest |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Capital invested |  |  |  |  |  |  |  | $ 14.5 |  |  |  |
HG Re |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  |  |  |  |  |  | 15.00% |  |  |  |  |
Restricted Cash |  |  | 222.8 |  |  |  | $ 221.2 |  |  |  |  |
BAM |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Principal |  | $ 800.0 | 75,287.7 |  |  |  | $ 77,689.3 |  |  | $ 1,100.0 |  |
Additional municipal bond guarantee |  |  |  |  |  |  |  |  |  | $ 36.9 |  |
BAM | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Repayments of unsecured debt |  |  | 30.1 | 65.0 |  |  |  |  |  |  |  |
Accrued interest payment on surplus notes |  |  | 8.4 | 16.2 |  |  |  |  |  |  |  |
BAM | Supplemental Trust |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Repayments of unsecured debt |  |  | 21.5 | 47.9 |  |  |  |  |  |  |  |
Accrued interest payment on surplus notes |  |  | $ 0.2 | $ 0.9 |  |  |  |  |  |  |  |
BAM | US Treasury (UST) Interest Rate | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  |  |  | 3.00% |  |  |  |  |
BAM | Fixed interest rate | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  |  |  |  |  |  | 8.00% |  |  |  |  |
BAM | Forecast | US Treasury (UST) Interest Rate | Surplus Note |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Effective percentage |  |  |  |  |  |  |  |  | 3.10% |  |  |
BAM | Fidus Re Ltd. |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt | $ 150.0 |  |  |  |  | $ 100.0 |  |  |  |  |  |
Aggregate losses on ceded business | 135.0 |  |  |  |  | 165.0 |  |  |  |  |  |
Premium receivable, Premium expected to be collected | 150.0 |  |  |  |  | 100.0 |  |  |  |  |  |
Percentage Of Financial Guaranty Policies Covered |  |  |  |  |  |  | 41.00% |  |  |  |  |
Principal |  |  |  |  | $ 2,200.0 |  |  |  |  |  |  |
Percentage of quota share reinsurance agreement |  | 100.00% |  |  | 100.00% |  |  |  |  |  |  |
BAM | Fidus Re Ltd. | Maximum |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Aggregate losses on ceded business | $ 301.7 |  |  |  |  | $ 276.1 |  |  |  |  |  |
Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Losses above retention, percentage of liability | 90.00% |  |  |  |  | 90.00% |  |  |  |  |  |

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