Share capital - Authorized and issued (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Equity [Abstract]
Authorized share capital	$ 33.0	$ 33.0
Common share, shares authorized (in shares)	300,000,000	300,000,000	300,000,000
Common share, par value (in dollars per share)	$ 0.10	$ 0.10	$ 0.10
Preference shares, shares authorized (in shares)	30,000,000	30,000,000	30,000,000
Preference shares, par value (in dollars per share)	$ 0.10	$ 0.10	$ 0.10
Common share, shares issued (in shares)	106,285,663	106,285,663	105,479,341
Common share, shares outstanding (in shares)	106,285,663	106,285,663	105,479,341	105,262,341	104,473,402
Preference shares, shares issued (in shares)	0	0	0
Preference shares, shares outstanding (in shares)	0	0	0
Stock Repurchased and Retired During Period, Shares	644,768	644,768
Payments for Repurchase of Common Stock	$ 7.4	$ 7.4
Treasury Stock Acquired, Average Cost Per Share	$ 11.46	$ 11.46