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Stockholders' equity (Details) - USD ($)
shares in Millions
 6 Months Ended
Jun. 30, 2015
Dec. 31, 2015
Sep. 30, 2015
Class of Stock [Line Items]      
Authorized amount     $ 300,000,000
Remaining authorized repurchase amount   $ 264,400,000  
Chief Executive Officer (CEO) [Member] | Common Class B [Member]      
Class of Stock [Line Items]      
Shares contributed to company without consideration 5.2