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Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 20, 2016
Class of Stock [Line Items] |  |  |  |  |
Ordinary shares, par value (in usd per share) |  |  | $ 0.0015144558 | $ 0.0015144558 |
Authorized share capital: |  |  |  |  |
Number of ordinary shares (in shares) |  |  | 969,629,030 | 969,629,030 |
Number of non-voting shares (in shares) |  |  | 6,787,880 | 6,787,880 |
Number of preference shares (in shares) |  |  | 100,000,000 | 100,000,000 |
Ordinary Shares 0.15144558¢ per share |  |  | $ 1,469 | $ 1,469 |
Non-Voting Shares 0.15144558¢ per share |  |  | 10 | 10 |
Preference Shares 0.15144558¢ per share |  |  | 152 | 152 |
Total authorized share capital |  |  | $ 1,631 | $ 1,631 |
Issued share capital: |  |  |  |  |
Ordinary shares, issued (in shares) |  |  | 59,697,669 | 59,474,085 |
Issued ordinary shares of 0.15144558¢ per share |  |  | $ 90 | $ 90 |
Total issued share capital |  |  | $ 122 | $ 122 |
Non Voting Shares Par Or Stated Value Per Share |  |  | $ 0.0015144558 | $ 0.0015144558 |
Preference shares, par value (in usd per share) |  |  | 0.0015144558 | 0.0015144558 |
Preferred shares, liquidation preference (in USD per share) |  |  | $ 25 | $ 25 |
Ordinary shares acquired and canceled, shares | 451,268 | 648,941 |  |  |
Ordinary shares purchased and canceled, value | $ 20,000 | $ 30,000 |  |  |
5.95% preference shares |  |  |  |  |
Issued share capital: |  |  |  |  |
Preference shares, issued (in shares) |  |  | 11,000,000 | 11,000,000 |
Issued preference shares of 0.15144558¢ each with a liquidation preference |  |  | $ 17 | $ 17 |
Preference shares, par value (in usd per share) |  |  | $ 0.0015144558 | $ 0.0015144558 |
Preferred shares, liquidation preference (in USD per share) |  |  | $ 25 | $ 25 |
Preferred Stock Dividends Rate Percentage |  |  | 5.95% | 5.95% |
5.625% preference shares |  |  |  |  |
Issued share capital: |  |  |  |  |
Preference shares, issued (in shares) |  |  | 10,000,000 | 10,000,000 | 10,000,000
Issued preference shares of 0.15144558¢ each with a liquidation preference |  |  | $ 15 | $ 15 |
Preference shares, par value (in usd per share) |  |  | $ 0.0015144558 | $ 0.0015144558 |
Preferred shares, liquidation preference (in USD per share) |  |  | $ 25 | $ 25 |
Preferred Stock Dividends Rate Percentage |  |  | 5.625% | 5.625% |

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