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Shareholders' equity (Details) (USD $)
In Millions, except Share data, unless otherwise specified
Dec. 31, 2013
Dec. 19, 2013
Mar. 20, 2013
Mar. 19, 2013
Equity [Abstract]        
Common stock reserved for future issuance 16,231,674      
Common stock reserved for future issuance in equity forward transaction 5,700,000      
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) $ 25.74688      
Initial fair value (in dollars per share) $ 0      
Delivery of net shares on settlement 188,000 1,300,000    
Delivery of cash on settlement $ 5.0 $ 32.1    
Underwriting discount included in delivery of cash 6.0 1.3    
Cash in exchange of physical delivery of shares on settlement $ 141