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Fair Value Measurements - Inputs Used (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gross written premiums $ 3,360.9 $ 3,147.0 $ 2,997.3
Minimum | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gross written premiums 50.1 38.9  
Reserve for losses 4.2 2.7  
Initial value of issuance 325.0 220.0  
Maximum | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gross written premiums 61.1 43.4  
Reserve for losses $ 61.9 $ 11.8  
Contract period 365 days 365 days  
Initial value of issuance $ 325.0 $ 220.0  
Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Initial value of issuance 44.2 115.0  
Liabilities measured using level 3 inputs classified as other payables 42.4 108.4  
Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Initial value of issuance 44.2 115.0  
Liabilities measured using level 3 inputs classified as other payables 42.4 108.4  
Silverton 2015 | Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value of Loan Notes   223.4 $ 190.6
Silverton 2016 | Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value of Loan Notes $ 86.6 $ (223.4)