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