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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Catastrophe bonds trading, at fair value | $ 37.7 | $ 33.5
Derivatives at fair value | 6.4 | 6.4
Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Catastrophe bonds trading, at fair value | 37.4 | 32.4
Loan notes issued by variable interest entities, at fair value |  | (44.2)
Total | 6,837.2 | 7,485.4
Fair Value, Measurements, Recurring | Short term investments, trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 4.3 | 73.0
Fair Value, Measurements, Recurring | Equity investments trading, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 491.0
Fair Value, Measurements, Recurring | Derivatives at fair value, assets | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 6.4 | 6.4
Fair Value, Measurements, Recurring | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities at fair value | (3.1) | (1.0)
Available for sale financial assets | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 5,176.4 | 5,231.0
Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 1,387.8 | 1,159.4
Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 46.4 | 52.1
Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 52.1 | 54.9
Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 2,266.0 | 2,415.7
Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 90.8 | 91.3
Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 405.4 | 484.9
Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 18.5 | 26.2
Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 909.4 | 946.5
Short-term investments | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 120.4 | 89.9
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 1,407.2 | 1,649.3
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 165.9 | 161.9
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 20.7 | 32.2
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 874.9 | 1,046.3
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 1.0
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 227.5 | 202.5
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 4.9 | 9.9
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 113.3 | 195.5
Real estate funds | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 100.5 |
Level 1 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Catastrophe bonds trading, at fair value | 0.0 | 0.0
Loan notes issued by variable interest entities, at fair value |  | 0.0
Total | 1,983.9 | 2,338.3
Level 1 | Fair Value, Measurements, Recurring | Short term investments, trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.5 | 73.0
Level 1 | Fair Value, Measurements, Recurring | Equity investments trading, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 491.0
Level 1 | Fair Value, Measurements, Recurring | Derivatives at fair value, assets | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 0.0 | 0.0
Level 1 | Fair Value, Measurements, Recurring | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities at fair value | 0.0 | 0.0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 1,658.2 | 1,500.6
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 1,387.8 | 1,159.4
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 270.4 | 341.2
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Short-term investments | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 109.9 | 87.3
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 215.3 | 186.4
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 165.9 | 161.9
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 49.4 | 24.5
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 1 | Real estate funds | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 |
Level 2 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Catastrophe bonds trading, at fair value | 37.4 | 32.4
Loan notes issued by variable interest entities, at fair value |  | 0.0
Total | 4,765.1 | 5,233.7
Level 2 | Fair Value, Measurements, Recurring | Short term investments, trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 3.8 | 0.0
Level 2 | Fair Value, Measurements, Recurring | Equity investments trading, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 2 | Fair Value, Measurements, Recurring | Derivatives at fair value, assets | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 6.4 | 6.4
Level 2 | Fair Value, Measurements, Recurring | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities at fair value | (3.1) | (1.0)
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 3,518.2 | 3,730.4
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 46.4 | 52.1
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 52.1 | 54.9
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 2,266.0 | 2,415.7
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 90.8 | 91.3
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 135.0 | 143.7
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 18.5 | 26.2
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 909.4 | 946.5
Level 2 | Short-term investments | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 10.5 | 2.6
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 1,191.9 | 1,462.9
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 20.7 | 32.2
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 874.9 | 1,046.3
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 1.0
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 178.1 | 178.0
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 4.9 | 9.9
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 113.3 | 195.5
Level 2 | Real estate funds | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 |
Level 3 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Catastrophe bonds trading, at fair value | 0.0 | 0.0
Loan notes issued by variable interest entities, at fair value |  | (44.2)
Total | (12.3) | (86.6)
Level 3 | Fair Value, Measurements, Recurring | Short term investments, trading at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Fair Value, Measurements, Recurring | Equity investments trading, at fair value |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Fair Value, Measurements, Recurring | Derivatives at fair value, assets | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivatives at fair value | 0.0 | 0.0
Level 3 | Fair Value, Measurements, Recurring | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Short-term investments | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 | 0.0
Level 3 | Real estate funds | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments, at fair value | 0.0 |
Accrued expenses and other payables | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loan notes issued by variable interest entities, at fair value | (12.3) | (42.4)
Accrued expenses and other payables | Level 1 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loan notes issued by variable interest entities, at fair value | 0.0 | 0.0
Accrued expenses and other payables | Level 2 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loan notes issued by variable interest entities, at fair value | 0.0 | 0.0
Accrued expenses and other payables | Level 3 | Fair Value, Measurements, Recurring |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Loan notes issued by variable interest entities, at fair value | $ (12.3) | $ (42.4)

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- Definition Catastrophe Bonds, Fair Value Disclosure + References No definition available. + Details
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Balance Type: | debit
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- Definition Debt Instrument, Face Amount, Issued By Third Parties + References No definition available. + Details
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Namespace Prefix: | ahl_
Data Type: | xbrli:monetaryItemType
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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