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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

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