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Employee benefit plans - Narrative (Details)
$ / shares in Units, $ in Millions
1 Months Ended 9 Months Ended
Jun. 30, 2014
shares
Sep. 30, 2016
USD ($)
plan
$ / shares
shares
Sep. 30, 2015
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of stock-based employee compensation plans | plan   3  
Unearned stock-based compensation, expected recognition period   2 years 10 months 3 days  
Employee Stock Ownership Plan (ESOP), Plan Description   P6M  
ESPP stock issued during period (shares)   668,107 436,924
ESPP weighted average purchase price of shares purchased (usd per share) | $ / shares   $ 9.15 $ 26.88
Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unearned stock-based compensation, expected recognition period   10 years  
Award vesting period   4 years  
RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period   4 years  
Shares granted (shares)   6,912,000  
Weighted average price of shares granted (usd per share) | $ / shares   $ 12.15  
RSUs [Member] | Chief Executive Officer (CEO) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted (shares) 4,500,000    
RSUs [Member] | Vested Immediately [Member] | Chief Executive Officer (CEO) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted (shares) 1,500,000    
Allocated share-based compensation expense | $   $ 5.5 $ 26.1
RSUs [Member] | Share-based Compensation Award, Tranche Two [Member] | Chief Executive Officer (CEO) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted (shares) 3,000,000    
Employee Stock Purchase Plan Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Purchase price of common stock as percentage of fair value of Company common stock (percent)   85.00%  
Weighted average price of shares granted (usd per share) | $ / shares   $ 4.23 $ 15.13
Stock Options, ESPP and Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unearned stock-based compensation costs | $   $ 164.2