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Capital Structure - Summary of Authorized and Issued Share Capital (Phantom) (Detail) - $ / shares | Dec. 31, 2017 | Jul. 03, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Class of Stock [Line Items] |  |  |
Ordinary shares, par value | $ 0.0015144558 |  | $ 0.0015144558
Non-voting shares, par value | 0.0015144558 |  | 0.0015144558
Preference shares, par value | 0.0015144558 |  | 0.0015144558
Redemption price per share | 25 |  | 25
5.950% Preference Shares |  |  |
Class of Stock [Line Items] |  |  |
Preference shares, par value | 0.0015144558 |  | 0.0015144558
Redemption price per share | $ 25 |  | $ 25
Preference shares, rate | 5.95% |  | 5.95%
7.401% Preference Shares |  |  |
Class of Stock [Line Items] |  |  |
Preference shares, par value |  |  | $ 0.0015144558
Redemption price per share |  | $ 25 | $ 25
Preference shares, rate |  |  | 7.401%
7.250% Preference Shares |  |  |
Class of Stock [Line Items] |  |  |
Preference shares, par value |  |  | $ 0.0015144558
Redemption price per share |  |  | $ 25
Preference shares, rate |  |  | 7.25%
5.625% Preference Shares |  |  |
Class of Stock [Line Items] |  |  |
Preference shares, par value | $ 0.0015144558 |  | $ 0.0015144558
Redemption price per share | $ 25 |  | $ 25
Preference shares, rate | 5.625% |  | 5.625%

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