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Earnings Per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 3 Months Ended 12 Months Ended
Jan. 03, 2017
Jul. 31, 2017
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share [Abstract]                                  
Net (loss)/income     $ (184.9) $ (253.8) $ 75.8 $ 96.5 $ (71.5) $ 95.6 $ 64.9 $ 114.4 $ 117.9 $ 28.2 $ 49.0 $ 128.0 $ (266.4) $ 203.4 $ 323.1
Preference share dividends                             (36.2) (41.8) (37.8)
Preference share redemption costs                             (8.0) 0.0 0.0
Net profit attributable to non-controlling interest                             (1.3) (0.1) (0.8)
Basic net income available to ordinary shareholders                             (311.9) 161.5 284.5
Diluted net income available to ordinary shareholders                             $ (311.9) $ 284.5 $ 161.5
Ordinary shares:                                  
Basic weighted average ordinary shares     59,431,469 59,759,730 59,966,358 59,862,662 60,152,420 60,225,705 60,705,028 60,867,815 60,784,832 60,779,295 61,408,633 62,159,303 59,753,886 60,478,740 61,287,884
Weighted average effect of dilutive securities                             0 1,381,949 1,399,619
Total diluted weighted average ordinary shares     59,431,469 59,795,730 61,022,981 61,196,772 61,198,041 61,577,018 62,192,142 62,483,938 62,176,505 62,155,125 62,896,907 63,532,662 59,753,886 61,860,689 62,687,503
Earnings per ordinary share:                                  
Basic, dollars per share     $ (3.25) $ (4.48) $ 1.09 $ 1.39 $ (1.41) $ 1.43 $ 0.91 $ 1.73 $ 1.78 $ 0.30 $ 0.64 $ 1.91 $ (5.22) $ 2.67 $ 4.64
Diluted, dollars per share     $ (3.25) $ (4.48) $ 1.07 $ 1.36 $ (1.41) $ 1.40 $ 0.89 $ 1.68 $ 1.75 $ 0.30 $ 0.62 $ 1.87 $ (5.22) $ 2.61 $ 4.54
7.401% Preference Shares                                  
Class of Stock [Line Items]                                  
Preferred Stock, Dividend Rate, Percentage 7.401%                           7.401% 7.401%  
7.250% Preference Shares                                  
Class of Stock [Line Items]                                  
Preferred Stock, Dividend Rate, Percentage   7.25%                         7.25% 7.25%  
Additional paid-in capital | Preference Shares                                  
Earnings Per Share [Abstract]                                  
Preference share redemption costs                             $ 8.0    
Class of Stock [Line Items]                                  
Preference shares redeemed and cancelled                             $ 293.2 $ 0.0 $ 0.0