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Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Underwriting Revenues        
Gross written premiums $ 1,116.8 $ 998.0    
Net written premiums 635.5 686.2    
Gross earned premiums 842.6 751.3    
Net earned premiums 533.5 581.1    
Underwriting Expenses        
Losses and loss adjustment expenses 310.2 328.2    
Amortization of deferred policy acquisition costs 90.8 113.7    
General and administrative expenses 95.2 105.7    
Underwriting income 37.3 33.5    
Corporate expenses (13.7) (13.4)    
Non-operating expenses (12.1) (2.2)    
Net investment income 47.3 47.7    
Realized and unrealized investment gains 100.6 51.2    
Realized and unrealized investment losses (138.3) (5.0)    
Change in fair value of loan notes issued by variable interest entities 1.0 (2.9)    
Change in fair value of derivatives 23.5 3.1    
Interest expense on long term debt (7.4) (7.4)    
Net realized and unrealized foreign exchange (losses) (4.7) (8.9)    
Other income 2.1 3.6    
Other expenses (1.2) 0.0    
Income from operations before income tax 34.4 99.3    
Net reserves for loss and loss adjustment expenses $ 5,068.1 $ 4,730.1 $ 5,234.3 $ 4,759.2
Ratios        
Loss ratio 58.10% 56.50%    
Policy acquisition expense ratio 17.00% 19.60%    
General and administrative expense ratio 22.70% 20.90%    
Expense ratio 39.70% 40.50%    
Combined ratio 97.80% 97.00%    
Non-operating expenses related to Effectiveness and Efficiency Program $ 11.8      
Reinsurance        
Underwriting Revenues        
Gross written premiums 623.5 $ 565.3    
Net written premiums 425.0 448.2    
Gross earned premiums 375.0 327.6    
Net earned premiums 282.5 277.5    
Underwriting Expenses        
Losses and loss adjustment expenses 166.9 143.1    
Amortization of deferred policy acquisition costs 55.9 59.5    
General and administrative expenses 31.6 43.9    
Underwriting income 28.1 31.0    
Net reserves for loss and loss adjustment expenses $ 2,823.6 $ 2,445.4    
Ratios        
Loss ratio 59.10% 51.60%    
Policy acquisition expense ratio 19.80% 21.40%    
General and administrative expense ratio 11.20% 15.80%    
Expense ratio 31.00% 37.20%    
Combined ratio 90.10% 88.80%    
Insurance        
Underwriting Revenues        
Gross written premiums $ 493.3 $ 432.7    
Net written premiums 210.5 238.0    
Gross earned premiums 467.6 423.7    
Net earned premiums 251.0 303.6    
Underwriting Expenses        
Losses and loss adjustment expenses 143.3 185.1    
Amortization of deferred policy acquisition costs 34.9 54.2    
General and administrative expenses 63.6 61.8    
Underwriting income 9.2 2.5    
Net reserves for loss and loss adjustment expenses $ 2,244.5 $ 2,284.7    
Ratios        
Loss ratio 57.10% 61.00%    
Policy acquisition expense ratio 13.90% 17.90%    
General and administrative expense ratio 25.30% 20.40%    
Expense ratio 39.20% 38.30%    
Combined ratio 96.30% 99.30%