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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 | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Class of Stock [Line Items] |  |  |  |
Net income |  | $ 30.8 | $ 96.5 |
Preference share dividends |  | (7.6) | (10.5) |
Change in redemption value |  | 0.0 | (2.4) |
Proportion due to non-controlling interest |  | (0.2) | (0.1) |
Basic and diluted net income available to ordinary shareholders |  | 23.0 | 83.5 |
Diluted net income available to ordinary shareholders |  | $ 23.0 | $ 83.5 |
Ordinary shares: |  |  |  |
Basic weighted average ordinary shares |  | 59,546,165 | 59,862,662 |
Weighted average effect of dilutive securities |  | 966,982 | 1,334,110 |
Total diluted weighted average ordinary shares |  | 60,513,147 | 61,196,772 |
Earnings per ordinary share: |  |  |  |
Basic (in usd per share) |  | $ 0.39 | $ 1.39 |
Diluted (in usd per share) |  | $ 0.38 | $ 1.36 |
7.401% preference shares |  |  |  |
Earnings per ordinary share: |  |  |  |
Preference shares, rate |  | 7.401% |  |
Retained earnings |  |  |  |
Class of Stock [Line Items] |  |  |  |
Net income |  | $ 30.8 | $ 96.5 |
Proportion due to non-controlling interest |  | (0.2) | (0.1) |
Retained earnings | Redemption costs of preference shares |  |  |  |
Class of Stock [Line Items] |  |  |  |
Preference share dividends |  | (7.6) | (10.5) |
Change in redemption value | [1] | 0.0 |  |
Additional paid-in capital | Redemption costs of preference shares |  |  |  |
Class of Stock [Line Items] |  |  |  |
Change in redemption value | [1] | 0.0 | 2.4 | $ (2.4)
Earnings per ordinary share: |  |  |  |
Stock Redeemed or Called During Period, Value |  | $ 0.0 | $ 133.2 |
[1] | The $2.4 million deduction from net income in 2017 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the 7.401% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the final redemption costs of $133.2 million.

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- Definition Amount of paid and unpaid preferred stock dividends declared with the form of settlement in cash, stock and payment-in-kind (PIK). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 405 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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