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Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Catastrophe bonds trading, at fair value $ 33.5 $ 42.5  
Derivatives at fair value 8.4 7.2  
Fair Value, Measurements, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Catastrophe bonds trading, at fair value 30.3    
Loan notes issued by variable interest entities, at fair value (101.5) (115.0)  
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) (2.4) (108.4)  
Total 7,555.6 7,653.6  
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 90.3 185.4  
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 468.6 584.7  
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 8.4 7.2  
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 under derivative contracts, fair value (2.9) (18.4)  
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 5,365.4 5,664.6  
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 1,172.6 1,206.1  
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 62.1 119.6  
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 50.1 24.4  
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 2,468.4 2,586.5  
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 94.0 89.8  
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 531.7 488.7  
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 33.7 63.0  
Available for sale financial assets | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, at fair value   12.6  
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 952.8 1,073.9  
Short-term investments | Fair Value, Measurements, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, at fair value 34.4 145.3  
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 1,665.0 1,265.7  
Catastrophe bonds trading, at fair value   42.5  
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 160.6 82.4  
Held for trading financial assets, at fair value | 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    
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 32.3 15.5  
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 1,045.0 820.6  
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 2.0    
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 210.9 202.8  
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 11.1 14.5  
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 203.1 129.9  
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    
Loan notes issued by variable interest entities, at fair value 0.0 0.0  
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) 0.0 0.0  
Total 2,318.9 2,411.6  
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 90.0 76.1  
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 468.6 584.7  
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 under derivative contracts, 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 1,547.8 1,549.8  
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 1,172.6 1,206.1  
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 375.2 343.7  
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 | Non-agency commercial mortgage-backed      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, at fair value   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 27.4 118.6  
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 185.1 82.4  
Catastrophe bonds trading, at fair value   0.0  
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 160.6 82.4  
Level 1 | Held for trading financial assets, at fair value | 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    
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    
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 24.5 0.0  
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 2 | Fair Value, Measurements, Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Catastrophe bonds trading, at fair value 30.3    
Loan notes issued by variable interest entities, at fair value 0.0 0.0  
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) 0.0 0.0  
Total 5,340.6 5,465.4  
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 0.3 109.3  
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 8.4 7.2  
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 under derivative contracts, fair value (2.9) (18.4)  
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 3,817.6 4,114.8  
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 62.1 119.6  
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 50.1 24.4  
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 2,468.4 2,586.5  
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 94.0 89.8  
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 156.5 145.0  
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 33.7 63.0  
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, at fair value   12.6  
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 952.8 1,073.9  
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 7.0 26.7  
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 1,479.9 1,183.3  
Catastrophe bonds trading, at fair value   42.5  
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 | U.S. agency      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, at fair value 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 32.3 15.5  
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 1,045.0 820.6  
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 2.0    
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 186.4 202.8  
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 11.1 14.5  
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 203.1 129.9  
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    
Loan notes issued by variable interest entities, at fair value (101.5) (115.0)  
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) (2.4) (108.4) $ (2.3)
Total 103.9 223.4  
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 under derivative contracts, 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 | Non-agency commercial mortgage-backed      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, at fair value   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  
Catastrophe bonds trading, at fair value   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 | U.S. agency      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Investments, at fair value 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    
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