Shareholders' Equity - Prospectus Supplement (Details)			3 Months Ended
	Mar. 08, 2017 shares	Mar. 07, 2017 £ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 $ / shares	Nov. 10, 2014 USD ($)
Class of Stock [Line Items]
New issues (shares) | shares		9,000,000	10,350,000
Proceeds from issuance of common stock			$ 288,100,000
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
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