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Employee benefit plans - Narrative (Details)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended
Jun. 30, 2014
shares
Mar. 31, 2018
USD ($)
plan
$ / shares
shares
Mar. 31, 2017
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Stock-based Employee Compensation Plans | plan   3  
Shares granted (shares)   122,000  
ESPP stock issued during period (shares)   632,046 625,811
ESPP weighted average purchase price of shares purchased (usd per share) | $ / shares   $ 4.78 $ 8.02
Weighted average price of shares granted (usd per share) | $ / shares   $ 6.04  
Unearned stock-based compensation, expected recognition period   1 year 9 months 25 days  
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    
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]      
Award Vesting Period 3 years    
Shares granted (shares) 3,000,000    
Allocated share-based compensation expense | $   $ 0.0 $ 0.5
Employee Stock Purchase Plan Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Purchase Price of Common Stock, Percent   85.00%  
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 | $   $ 65.4  
2014 Equity Incentive Plans [Member] | Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expiration Period   10 years  
2014 Equity Incentive Plans [Member] | Minimum [Member] | Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award Vesting Period   1 year  
2014 Equity Incentive Plans [Member] | Minimum [Member] | RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award Vesting Period   2 years  
2014 Equity Incentive Plans [Member] | Maximum [Member] | Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award Vesting Period   4 years  
2014 Equity Incentive Plans [Member] | Maximum [Member] | RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award Vesting Period   4 years