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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Catastrophe Bonds, Fair Value Disclosure | $ 5.1 | $ 5.3
Derivatives at fair value | 56.2 | 12.9
Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total | 6,100.9 | 6,512.6
Recurring | Derivatives at fair value | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 56.2 | 12.9
Recurring | Liabilities under derivative contracts | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | (3.2) | (13.3)
Recurring | Liabilities under derivative contracts | Loss Portfolio Transfer |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | (31.7) |
Recurring | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 5.6 |
Recurring | Fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 3,788.6 | 4,881.6
Recurring | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 52.0 | 10.1
Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 1,475.5 | 1,157.5
Privately-held Investments, at fair value | 533.0 | 307.1
Catastrophe Bonds, Fair Value Disclosure | 2.9 | 3.4
Recurring | Short-term investments trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 6.3 | 2.0
Recurring | Real estate fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 221.3 | 151.3
Recurring | Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total | 1,428.7 | 1,384.5
Recurring | Level 1 | Derivatives at fair value | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 0.0 | 0.0
Recurring | Level 1 | Liabilities under derivative contracts | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | 0.0 | 0.0
Recurring | Level 1 | Liabilities under derivative contracts | Loss Portfolio Transfer |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | 0.0 |
Recurring | Level 1 | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 |
Recurring | Level 1 | Fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 1,098.3 | 1,248.7
Recurring | Level 1 | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 51.9 | 6.5
Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 272.2 | 128.2
Privately-held Investments, at fair value | 0.0 | 0.0
Catastrophe Bonds, Fair Value Disclosure | 0.0 | 0.0
Recurring | Level 1 | Short-term investments trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 6.3 | 1.1
Recurring | Level 1 | Real estate fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Recurring | Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total | 3,949.6 | 4,669.7
Recurring | Level 2 | Derivatives at fair value | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 56.2 | 12.9
Recurring | Level 2 | Liabilities under derivative contracts | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | (3.2) | (13.3)
Recurring | Level 2 | Liabilities under derivative contracts | Loss Portfolio Transfer |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | 0.0 |
Recurring | Level 2 | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 5.6 |
Recurring | Level 2 | Fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 2,690.3 | 3,632.9
Recurring | Level 2 | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.1 | 3.6
Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 1,203.3 | 1,029.3
Privately-held Investments, at fair value | 0.0 | 0.0
Catastrophe Bonds, Fair Value Disclosure | 2.9 | 3.4
Recurring | Level 2 | Short-term investments trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.9
Recurring | Level 2 | Real estate fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Recurring | Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total | 501.3 | 307.1
Recurring | Level 3 | Derivatives at fair value | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 0.0 | 0.0
Recurring | Level 3 | Liabilities under derivative contracts | Foreign exchange contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | 0.0 | 0.0
Recurring | Level 3 | Liabilities under derivative contracts | Loss Portfolio Transfer |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities under derivative contracts, fair value | (31.7) |
Recurring | Level 3 | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 |
Recurring | Level 3 | Fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Recurring | Level 3 | Short-term investments |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Privately-held Investments, at fair value | 533.0 | 307.1
Catastrophe Bonds, Fair Value Disclosure | 0.0 | 0.0
Recurring | Level 3 | Short-term investments trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Recurring | Level 3 | Real estate fund |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
U.S. government | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 953.0 | 1,084.3
U.S. government | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 261.6 | 116.3
U.S. government | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 953.0 | 1,084.3
U.S. government | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 261.6 | 116.3
U.S. government | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
U.S. government | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
U.S. government | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
U.S. government | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
U.S. agency | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 8.8 | 21.5
U.S. agency | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
U.S. agency | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 8.8 | 21.5
U.S. agency | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Municipal | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 149.5 | 82.6
Municipal | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 3.6 | 4.0
Municipal | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Municipal | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Municipal | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 149.5 | 82.6
Municipal | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 3.6 | 4.0
Municipal | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Municipal | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Corporate | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 1,845.0 | 2,208.3
Corporate | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 162.1 | 96.8
Corporate | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Corporate | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Corporate | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 1,845.0 | 2,208.3
Corporate | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 162.1 | 96.8
Corporate | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Corporate | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Non-U.S. government-backed corporate | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 110.4 | 136.8
Non-U.S. government-backed corporate | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 11.6 | 13.1
Non-U.S. government-backed corporate | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Non-U.S. government-backed corporate | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Non-U.S. government-backed corporate | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 110.4 | 136.8
Non-U.S. government-backed corporate | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 11.6 | 13.1
Non-U.S. government-backed corporate | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Non-U.S. government-backed corporate | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Non-U.S government | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 213.6 | 250.7
Non-U.S government | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 30.4 | 34.5
Non-U.S government | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 145.3 | 164.4
Non-U.S government | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 10.6 | 11.9
Non-U.S government | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 68.3 | 86.3
Non-U.S government | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 19.8 | 22.6
Non-U.S government | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Non-U.S government | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Asset-backed | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.5
Asset-backed | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 896.5 | 785.7
Asset-backed | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Asset-backed | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Asset-backed | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.5
Asset-backed | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 896.5 | 785.7
Asset-backed | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Asset-backed | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Non-agency commercial mortgage-backed securities | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value |  | 6.9
Non-agency commercial mortgage-backed securities | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value |  | 0.0
Non-agency commercial mortgage-backed securities | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value |  | 6.9
Non-agency commercial mortgage-backed securities | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value |  | 0.0
Agency mortgage-backed | Recurring | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 502.7 | 1,090.0
Agency mortgage-backed | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 21.4 | 30.2
Agency mortgage-backed | Recurring | Level 1 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Agency mortgage-backed | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Agency mortgage-backed | Recurring | Level 2 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 502.7 | 1,090.0
Agency mortgage-backed | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 21.4 | 30.2
Agency mortgage-backed | Recurring | Level 3 | Total fixed income securities — Available for sale |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
Agency mortgage-backed | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
High yield loans | Recurring | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 88.3 | 76.9
High yield loans | Recurring | Level 1 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 0.0 | 0.0
High yield loans | Recurring | Level 2 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | 88.3 | 76.9
High yield loans | Recurring | Level 3 | Held for trading financial assets, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, fair value | $ 0.0 | $ 0.0

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- Definition Catastrophe Bonds, Fair Value Disclosure + References No definition available. + Details
Name: | ahl_CatastropheBondsFairValueDisclosure
Namespace Prefix: | ahl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Privately-held Investments, at fair value + References No definition available. + Details
Name: | ahl_PrivatelyheldInvestmentsatfairvalue
Namespace Prefix: | ahl_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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