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Shareholders' Equity - Prospectus Supplement (Details) | Mar. 13, 2017 USD ($) shares | Mar. 08, 2017 shares | Mar. 07, 2017 £ / shares shares | Jun. 30, 2017 $ / shares | Dec. 31, 2016 $ / shares | Nov. 10, 2014 USD ($)
--- | --- | --- | --- | --- | --- | ---
Class of Stock [Line Items] |  |  |  |  |  |
New issues (shares) | shares | 10,350,000 |  | 9,000,000 |  |  |
Common stock, par value (in dollars per share) | (per share) |  |  | £ 0.01 | $ 0.01 | $ 0.01 |
Sale of stock, price per share (in dollars per share) | £ / shares |  |  | 27.8 |  |  |
Proceeds from issuance of common stock | $ 288,100,000 |  |  |  |  |
Maximum common stock value authorized under prospectus supplement |  |  |  |  |  | $ 150,000,000
Over-Allotment Option |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Proceeds from issuance of common stock | $ 37,600,000 |  |  |  |  |
Underwriters |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
Sale of stock, price per share (in dollars per share) | £ / shares |  |  | £ 26.7 |  |  |
Underwriters | Over-Allotment Option |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |
New issues (shares) | shares |  | 1,350,000 |  |  |  |
Additional purchase of shares, exercise term |  |  | 30 days |  |  |

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