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Fair Value Measurements - Inputs Used (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gross written premiums $ 3,446.9 $ 3,360.9 $ 3,147.0
Amount classified within accrued expenses and other payables 5.7 52.5  
Minimum | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gross written premiums 50.1 50.1  
Reserve for losses 4.2 4.2  
Initial value of issuance 325.0 325.0  
Maximum | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Gross written premiums 61.1 61.1  
Reserve for losses 61.9 61.9  
Initial value of issuance 325.0 325.0  
Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Initial value of issuance 0.0 44.2  
Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Initial value of issuance 0.0 44.2  
Silverton 2017 | Recurring | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Fair value of Loan Notes $ 4.6 86.6  
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
Measurement Input, Expected Term [Member] | Maximum | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contract period 365 days 365 days  
Third party      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Amount classified within accrued expenses and other payables $ 4.6 $ 42.4