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Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Authorized share capital: |  |  |
Number of ordinary shares | 969,629,030 | 969,629,030 |
Number of non-voting shares | 6,787,880 | 6,787,880 |
Number of preference shares | 100,000,000 | 100,000,000 |
Ordinary Shares, authorized | $ 1,469 | $ 1,469 |
Non-Voting Shares, authorized | 10 | 10 |
Preference Shares, authorized | 152 | 152 |
Total authorized share capital | $ 1,631 | $ 1,631 |
Issued share capital: |  |  |
Ordinary shares, issued | 59,474,085 | 59,774,464 | 60,918,373
Ordinary shares, value | $ 90 | $ 91 |
Total issued share capital | $ 122 | $ 140 |
7.401% Preference Shares |  |  |
Issued share capital: |  |  |
Preference shares, issued | 0 | 5,327,500 |
Preference shares, value | $ 0 | $ 8 |
7.250% Preference Shares |  |  |
Issued share capital: |  |  |
Preference shares, issued | 0 | 6,400,000 |
Preference shares, value | $ 0 | $ 10 |
5.950% Preference Shares |  |  |
Issued share capital: |  |  |
Preference shares, issued | 11,000,000 | 11,000,000 |
Preference shares, value | $ 17 | $ 17 |
5.625% Preference Shares |  |  |
Issued share capital: |  |  |
Preference shares, issued | 10,000,000 | 10,000,000 |
Preference shares, value | $ 15 | $ 15 |

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