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Investments - Additional Information (Narrative) (Details) |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |  |
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Jan. 01, 2017 USD ($) | Mar. 31, 2021 USD ($) | Dec. 31, 2024 USD ($) security | Dec. 31, 2023 USD ($) security | Dec. 31, 2022 USD ($) | Dec. 31, 2024 GBP (£) security | Aug. 01, 2024 USD ($) | Dec. 31, 2023 GBP (£) security | Sep. 30, 2023 USD ($) | Aug. 31, 2023 GBP (£) | Apr. 01, 2023 USD ($) | May 05, 2022 USD ($) | Apr. 01, 2022 USD ($) | Sep. 30, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 23, 2019 USD ($) | Dec. 20, 2017 USD ($)
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Catastrophe bonds, trading at fair value (amortized cost — 2024: $1.0 and 2023: $1.6) |  |  |  | $ 1,000,000.0 | $ 1,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Market Value |  |  |  | 1,488,700,000 | 1,964,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0 | 400,000 | $ 2,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |
Other Investments | [1] |  |  | 267,200,000 | $ 209,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment funded |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions |  |  |  | (1,170) | (950) |  | (1,170) |  | (950) |  |  |  |  |  |  |  |  |
Total, Gross Unrealized Losses |  |  |  | $ (181,900,000) | $ (221,800,000) |  |  |  |  |  |  |  |  |  |  |  |  |
Securities Below Investment Grade or NR |  |  |  |  | 1,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Real Estate (2021)Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  |  | 900,000 | 2,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Apollo Real Estate Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  |  | 3,700,000 | 4,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Energy Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  |  | 55,000,000.0 | 55,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Infrastructure fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  |  | 2,300,000 | 4,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Lloyd's Enhanced Liquidity Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Managed Debt Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment | £ |  |  |  |  |  |  | £ 6,800,000 |  | £ 6,900,000 |  |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Managed lending fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  |  | 200,000 | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 111,700,000 | 117,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Managed Lending Fund (Dec 2020) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 19,300,000 | 15,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Pledge Accounts, Custodian Bank (April 2021) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 2,000,000 | 1,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Real estate fund (Sept 2021) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 36,200,000 | 39,800,000 |  |  |  |  |  |  |  |  | $ 10,000,000.0 |  |  |  |
Fair Value, Recurring [Member] | Real estate fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 267,200,000 | 209,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Apollo real estate fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 19,300,000 | 23,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Infrastructure fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 12,900,000 | 10,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Managed Debt Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 100,000 | 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Lloyd's enhanced liquidity fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 25,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Five Times Square |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | 40,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Limited Partner |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,000,000.0 |  |  | $ 100,000,000.0
Limited Partner | Managed lending fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Assets |  |  |  |  |  |  |  |  |  |  |  | $ 2,800,000 |  |  |  | $ 10,500,000 |  |
Limited Partner | Real estate fund | Apollo real estate fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Assets |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,000,000.0 |  |  |  |
Limited Partner | Infrastructure fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Assets |  |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000.0 |  |  |  |  |
Limited Partner | Managed Debt Fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Assets | £ |  |  |  |  |  |  |  |  |  |  | £ 7,000,000.0 |  |  |  |  |  |  |
Limited Partner | Third party energy fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Assets |  |  |  |  |  |  |  |  |  | $ 55,000,000.0 |  |  |  |  |  |  |  |
Limited Partner | Lloyd's enhanced liquidity fund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted Assets |  |  |  |  |  |  |  | $ 25,000,000.0 |  |  |  |  |  |  |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  | $ 800,000 |  | 0 | $ 400,000 |  |  |  |  |  |  |  |  |  |  |  |
Digital Re |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  | $ 2,300,000 |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  | 49.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Multi-Line Insurer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0 | 400,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 5,000,000.0 |
Commercial mortgage loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Market Value |  |  |  | 79,700,000 | 274,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Middle market loans and other private debt |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair Market Value |  |  |  | $ 61,000,000.0 | $ 84,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Other investments includes related party investments in Funds Managed by Apollo of $78.6 million (2023 — $39.8 million).

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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-6 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479106/326-30-50-4 + Details
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