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Municipal Bond Guarantee - Narrative (Details) $ in Millions | 1 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2019 USD ($) Contract | Apr. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) yr Contract | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) Contract | Dec. 31, 2017 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2019 | Jul. 31, 2012 USD ($)
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 50.00% |  | 50.00% |  | 50.00% |  |  |  |
Repayments of debt |  |  | $ 22.2 | $ 11.2 |  |  |  |  |
Interest paid |  |  | 11.7 | $ 5.3 |  |  |  |  |
Percentage of discounted future cash flows |  |  |  |  |  | 10.00% |  |  |
HG Global |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Partners capital contribution |  |  |  |  |  |  | $ 594.5 |  |
Surplus notes | $ 481.3 |  | $ 481.3 |  |  |  |  |  | $ 503.0
Assets held-in-trust |  |  |  |  | $ 603.0 |  |  |  |
HG Global | Preferred Stock |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 96.90% |  | 96.90% |  | 96.90% |  |  |  |
HG Global | Common Stock |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 88.40% |  | 88.40% |  | 88.40% |  |  |  |
BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Percentage of risk premium ceded | 60.00% |  | 60.00% |  |  |  |  |  |
Year of U.S. Treasury rate | yr |  |  | 1 |  |  |  |  |  |
Contracts outstanding | Contract | 8,508 |  | 8,508 |  | 7,525 |  |  |  |
Remaining weighted average contract period outstanding (in years) |  |  | 10 years 8 months 12 days |  | 10 years 8 months 12 days |  |  |  |
Municipal bond outstanding principal | $ 59,190.9 |  | $ 59,190.9 |  | $ 52,201.6 |  |  |  |
Municipal bond outstanding interest | 30,701.7 |  | 30,701.7 |  | 26,560.3 |  |  |  |
Municipal bond debt service outstanding | 89,892.6 |  | 89,892.6 |  | 78,761.9 |  |  |  |
October 1, 2019 - December 31, 2019 | 4.9 |  | 4.9 |  |  |  |  |  |
2020 | 18.7 |  | 18.7 |  |  |  |  |  |
2021 | 17.4 |  | 17.4 |  |  |  |  |  |
2022 | 16.3 |  | 16.3 |  |  |  |  |  |
2023 | 15.3 |  | 15.3 |  |  |  |  |  |
2024 | 14.3 |  | 14.3 |  |  |  |  |  |
2025 and thereafter | 116.6 |  | 116.6 |  |  |  |  |  |
Total gross unearned insurance premiums | 203.5 |  | 203.5 |  | 176.0 |  |  |  |
HG Global |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Collateral held in supplement trust | 787.4 |  | 787.4 |  | 757.4 |  |  |  |
Interest receivable | 164.2 |  | $ 164.2 |  | 143.7 |  |  |  |
HG Re |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured |  |  | 15.00% |  |  |  |  |  |
BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Surplus notes | $ 481.3 |  | $ 481.3 |  | $ 481.3 |  |  |  |
BAM Management |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Partners capital contribution |  |  |  |  |  |  | $ 14.5 |  |
Surplus Note | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  |  |  |  | 4.60% |  |  |  |
Surplus Note | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Repayments of debt |  |  |  |  | $ 17.7 |  |  |  |
Interest paid |  |  |  |  | $ 5.3 |  |  |  |
Combination One | Surplus Note | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Number of years of change in interest rate | yr |  |  | 5 |  |  |  |  |  |
US Treasury Rate | Surplus Note | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 3.00% |  |  |  |  |  |
Fixed interest rate | Surplus Note | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) | 8.00% |  | 8.00% |  | 8.00% |  |  |  |
First Quarter, year two | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
2020 | $ 4.8 |  | $ 4.8 |  |  |  |  |  |
Second Quarter, year two | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
2020 | 4.8 |  | 4.8 |  |  |  |  |  |
Third Quarter, year two | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
2020 | 4.6 |  | 4.6 |  |  |  |  |  |
Fourth Quarter, year two | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
2020 | 4.5 |  | $ 4.5 |  |  |  |  |  |
Series A BAM Surplus Note | HG Global |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  | $ 203.0 |  |  |
Series B BAM Surplus Note | HG Global |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  | $ 300.0 |  |  |
BAM | Fidus Re Ltd. |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Losses above retention, percentage of liability |  | 90.00% |  |  |  |  |  |  |
Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Secured Debt |  | $ 100.0 |  |  |  |  |  |  |
Collateralized financial guarantee, term |  | 12 years |  |  |  |  |  |  |
Collateral financial guarantee, callable term |  |  |  |  | 5 years |  |  |  |
Aggregate losses on ceded business |  | $ 165.0 |  |  |  |  |  |  |
Premium receivable, Premium expected to be collected |  | 100.0 |  |  |  |  |  |  |
Percentage of Quota Share Reinsurance Agreement |  |  |  |  | 100.00% |  |  |  |
Direct written premiums | $ 1,100.0 |  |  |  | $ 2,200.0 |  |  |  |
Minimum | Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Aggregate losses on ceded business |  | 165.0 |  |  |  |  |  |  |
Maximum | Fidus Re Ltd. | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Aggregate losses on ceded business |  | $ 276.1 |  |  |  |  |  |  |
Scenario, Forecast | Surplus Note | BAM |  |  |  |  |  |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |  |  |  |  |  |
Derivative, fixed interest rate (as a percent) |  |  |  |  |  |  |  | 5.70% |

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