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Stockholders' equity (Details) (USD $)
Share data in Millions, unless otherwise specified
3 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Shares authorized to be issued to employees under 2007 Plan (in shares) 29  
Shares available for grant (in shares) 4.0  
Compensation expense $ 14,660,000 $ 12,948,000
Compensation expense, net of tax $ 9,100,000 $ 9,300,000
Minimum [Member]
   
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Share-based compensation arrangement by share-based payment award, award vesting period 3 years  
Maximum [Member]
   
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Share-based compensation arrangement by share-based payment award, award vesting period 5 years  
Exercise period from date of issuance for stock option grants 10 years