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Capital Structure - Narrative (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Feb. 08, 2017
Class of Stock [Line Items] |  |  |  |  |
Additional paid-in capital |  | $ 966,000,000 |  | $ 954,700,000 |
Aggregate liquidation preferences |  | 525,000,000 |  | 525,000,000 |
Issuance costs |  | $ 13,100,000 |  | $ 13,100,000 |
Ordinary shares acquired and canceled, shares | 451,268 |  | 648,941 |  |
Ordinary shares purchased and canceled, value | $ 20,000,000 |  | $ 30,000,000 |  |
Ordinary shares acquired and canceled, average price per share (in usd per share) | $ 44.32 |  | $ 46.23 |  |
Repurchase Program 2017 |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Existing share repurchase authorization amount |  |  |  |  | $ 250,000,000
7.250% preference shares |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Preferred Stock, Dividend Rate, Percentage |  | 7.25% |  |  |

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- Definition Excess of issue price over par or stated value of the entity's capital stock and amounts received from other transactions involving the entity's stock or stockholders. Includes adjustments to additional paid in capital. Some examples of such adjustments include recording the issuance of debt with a beneficial conversion feature and certain tax consequences of equity instruments awarded to employees. Use this element for the aggregate amount of additional paid-in capital associated with common and preferred stock. For additional paid-in capital associated with only common stock, use the element additional paid in capital, common stock. For additional paid-in capital associated with only preferred stock, use the element additional paid in capital, preferred stock. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(30)(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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