Shareholders' equity Additional Information (Details) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Jul. 14, 2014	Dec. 19, 2013	Mar. 20, 2013	Mar. 19, 2013
Equity [Abstract]
Common stock reserved for future issuance	18,372,187
Common stock reserved for future issuance in equity forward transaction	4,700,000
Public offering related to equity forward transaction (in shares)					6,100,000
Sale of common stock related to equity forward transaction (in dollars per share)					$ 26.75
Closing price of common stock (in dollar per share)					$ 27.01
Offering related to underwriters exercising their over-allotment option under equity forward transaction (in shares)				900,000
Shares borrowed by forward counterparty from third party					7,000,000
Underwriting discount (in dollars per share)					$ 1.00312
Forward sale price (in dollars per share)	$ 22.63				$ 25.74688
Initial fair value (in dollars per share)	$ 0
Delivery of net shares on settlement	1,500,000	1,000,000	1,300,000
Delivery of cash on settlement	$ 51.0	$ 23.9	$ 32.1
Underwriting discount included in delivery of cash	5.0	1.0	1.3
Cash in exchange of physical delivery of shares on settlement	$ 106