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Shareholders' equity Additional Information (Details) - USD ($)
Dec. 31, 2015
Mar. 20, 2015
Jul. 14, 2014
Dec. 19, 2013
Mar. 20, 2013
Mar. 19, 2013
Equity [Abstract]            
Common stock reserved for future issuance 13,296,268          
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)   $ 4,700,000       $ 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 (in shares)   4,700,000 1,000,000 1,300,000    
Delivery of cash on settlement   $ 104,500,000 $ 23,900,000 $ 32,100,000    
Underwriting discount included in delivery of cash     $ 1,000,000 $ 1,300,000