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Employee benefit plans - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share-based compensation expense   $ 37,185,000 $ 41,854,000
ESPP stock issued during period (shares) 1,000,000 958,000 981,000
ESPP weighted average purchase price of shares purchased (usd per share) $ 3.42 $ 4.13 $ 4.78
Weighted average price of shares granted (usd per share) $ 4.59 $ 5.70 $ 5.83
Unearned stock-based compensation, expected recognition period 2 years 2 months 12 days    
Share-based Payment Arrangement, Expense, Tax Benefit $ 0 $ 0 $ 0
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 4.00%    
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 100.00%    
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent 100.00%    
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent 100.00%    
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 1,400,000 $ 4,000,000.0 $ 4,300,000
RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted (shares) 8,759,000    
Weighted average price of shares granted (usd per share) $ 4.59    
Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares granted (shares) 1,231,000    
Weighted average price of shares granted (usd per share) $ 4.05 $ 7.51 $ 5.76
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 $ 47,700,000    
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] | Performance Shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award Vesting Period 3 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