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Earnings per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Class of Stock [Line Items] |  |  |  |  |  |  |
Net (loss)/income | $ (253.8) |  | $ 95.6 | $ (81.5) |  | $ 274.9 |
Preference share dividends | (7.7) |  | (9.5) | (28.7) |  | (28.4) |
Change in redemption value | (5.6) |  | 0.0 | (8.0) |  | 0.0 |
Proportion due to non-controlling interest | (0.6) |  | 0.2 | (0.8) |  | 0.0 |
Basic and diluted net (loss) income available to ordinary shareholders | (267.7) |  | 86.3 | (119.0) |  | 246.5 |
Diluted net income available to ordinary shareholders | $ (267.7) |  | $ 86.3 | $ (119.0) |  | $ 246.5 |
Ordinary shares: |  |  |  |  |  |  |
Basic weighted average ordinary shares | 59,759,730 | [1] | 60,225,705 | 59,862,540 | [1] | 60,588,307 |
Weighted average effect of dilutive securities | 0 |  | 1,351,313 | 0 |  | 1,455,133 |
Total diluted weighted average ordinary shares | 59,759,730 | [1] | 61,577,018 | 59,862,540 | [1] | 62,043,440 |
(Loss) earnings per ordinary share: |  |  |  |  |  |  |
Basic (in usd per share) | $ (4.48) |  | $ 1.43 | $ (1.99) |  | $ 4.07 |
Diluted (in usd per share) | $ (4.48) |  | $ 1.40 | $ (1.99) |  | $ 3.97 |
Payment for preferred shares redemption |  |  |  | $ 293.2 |  | $ 0.0 |
7.401% preference shares |  |  |  |  |  |  |
(Loss) earnings per ordinary share: |  |  |  |  |  |  |
Preference shares, rate |  |  |  | 7.401% |  |  | 7.401%
7.250% preference shares |  |  |  |  |  |  |
(Loss) earnings per ordinary share: |  |  |  |  |  |  |
Preference shares, rate |  |  |  | 7.25% |  |  | 7.25%
[1] | The basic and diluted number of ordinary shares for the three and nine months ended September 30, 2017 is the same as the inclusion of dilutive securities in a loss making period would be anti-dilutive.

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