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Insurance Regulatory Restrictions Additional Details (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Average Annual Rate Of Return | 4.56% | 5.43%
Prescribed Discount Rate Percentage | 5.10% |
Quarterly Dividend Increase Limitation Percentage | 15.00% |
WISCONSIN |  |
Percentage Of Policy Holders Surplus | 10.00% |
ARIZONA |  |
Percentage Of Policy Holders Surplus | 10.00% |
Minimum [Member] |  |
Settlement Agreement Annual Restricted Payment Amount | $ 5 |
Maximum [Member] |  |
Settlement Agreement Annual Restricted Payment Amount | 8 |
Ambac Assurance [Member] |  |
Statutory Accounting Practices, Statutory Capital and Surplus, Balance | 865 | $ 1,088
Ambac Assurance [Member] | Prescribed Or Permitted Additional Accounting Practices [Member] |  |
Statutory Accounting Practices, Prescribed Practice, Amount | 40 | $ (12)
Everspan Indemnity |  |
Statutory Accounting Practices, Statutory Capital and Surplus, Balance | $ 26 |
5.1 % Surplus Notes Due 2020 [Member] | Ambac Assurance Corporation [Member] |  |
Debt Instrument, Interest Rate, Stated Percentage | 5.10% | 5.10%

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