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Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Gross written premiums $ 1,116.8 $ 998.0    
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 $ 64.5   $ 61.9  
Contract period 365 days   365 days  
Loan notes held by third parties $ 325.0   $ 325.0  
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  
Loan notes held by third parties 325.0   325.0  
Fair Value, Measurements, Recurring        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Total 7,139.7   7,485.4  
Loan notes held by third parties 32.2   44.2  
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) (7.6)   (42.4)  
Fair Value, Measurements, Recurring | Level 3        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Total (39.8)   (86.6)  
Loan notes held by third parties 32.2   44.2  
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) (7.6)   (42.4)  
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value   115.1   $ 223.4
Total     86.6  
Amount of Level 3 liabilities settled   111.2    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)   $ 2.9    
Fair Value, Measurements, Recurring | Level 3 | Silverton 2017        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value 39.8   $ 86.6  
Total 39.8      
Amount of Level 3 liabilities settled 45.8      
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) $ (1.0)