Employee benefit plans - Narrative (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 10,029,000	$ 8,869,000	$ 5,876,000	$ 7,637,000
ESPP stock issued during period (shares)					500,000	600,000
ESPP weighted average purchase price of shares purchased (usd per share)					$ 4.15	$ 3.38
Unearned stock-based compensation, expected recognition period					2 years 1 month 6 days
Share-based Payment Arrangement, Expense, Tax Benefit	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	$ 1,400,000	$ 4,000,000.0
RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (shares)					3,457,000
Weighted average price of shares granted (usd per share)					$ 8.22
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (shares)					573,000
Weighted average price of shares granted (usd per share)					$ 7.93
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	$ 60,100,000				$ 60,100,000
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