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Stockholders' equity (deficit) and redeemable convertible preferred stock - Treasury stock (Details) - USD ($)
$ / shares in Units, shares in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2015
Sep. 30, 2015
Class of Stock Disclosures [Abstract]      
Stock Repurchase Program, Authorized Amount     $ 300,000,000
Treasury Stock, Shares, Acquired 1.5    
Treasury Stock Acquired, Average Cost Per Share $ 23.05    
Repurchases of outstanding Class A common stock $ 35,600,000 $ 35,613,000