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Income Taxes - Additional Information (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Mar. 31, 2018
Income Taxes [Line Items] |  |  |
United States corporate tax at a rate |  | 21.00% |
Less valuation allowance | $ 138,900,000 | $ 149,200,000 |
Increase (decrease) in valuation allowance | (20,900,000) | 9,200,000 |
US branch profits, subject to tax | 0 | 0 |
Internal Revenue Service (IRS) |  |  |
Income Taxes [Line Items] |  |  |
Net operating loss carryforwards | 501,700,000 | 606,800,000 |
Operating loss carryforwards, remaining | 501,700,000 |  |
Operating loss carryforwards, subject to expiration date | 444,300,000 |  |
Operating loss carryforwards, not subject to expiration | 57,400,000 |  |
Capital loss carryforwards | 0 | 500,000 |
Charitable contribution carryforwards | 1,000,000.0 | 1,100,000 |
Less valuation allowance | 113,900,000 | 134,800,000 |
Internal Revenue Service (IRS) | Aspen U.K. |  |  |
Income Taxes [Line Items] |  |  |
Operating loss carryforwards, subject to s382 limitation | 6,500,000 |  |
Internal Revenue Service (IRS) | U.S. operating subsidiaries |  |  |
Income Taxes [Line Items] |  |  |
Operating loss carryforwards, subject to s382 limitation | $ 20,800,000 |  |
Internal Revenue Service (IRS) | Minimum |  |  |
Income Taxes [Line Items] |  |  |
Operating loss carryforwards, date of expiration | 2030 |  |
Internal Revenue Service (IRS) | Maximum |  |  |
Income Taxes [Line Items] |  |  |
Operating loss carryforwards, date of expiration | 2039 |  |
U.K. |  |  |
Income Taxes [Line Items] |  |  |
U. K. corporate tax rate reduced during the period |  |  | 19.00%
Net operating loss carryforwards | $ 99,500,000 | 93,200,000 |
Capital loss carryforwards | 3,800,000 |  |
Less valuation allowance | 24,800,000 | 14,400,000 |
Increase (decrease) in valuation allowance | 10,400,000 | 28,200,000 |
U.K. | Other Comprehensive Income |  |  |
Income Taxes [Line Items] |  |  |
Less valuation allowance | $ 0 | 0 |
Bermuda |  |  |
Income Taxes [Line Items] |  |  |
Bermuda tax rate | 0.00% |  |
Valuation Allowance, Operating Loss Carryforwards |  |  |
Income Taxes [Line Items] |  |  |
Increase (decrease) in valuation allowance | $ (20,900,000) | 9,200,000 |
Valuation Allowance, Operating Loss Carryforwards | U.K. |  |  |
Income Taxes [Line Items] |  |  |
Less valuation allowance | $ 10,400,000 | $ 28,200,000 |

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