Stock incentive plans - Restricted stock unit awards (Details)	9 Months Ended
	Sep. 30, 2020 $ / shares shares
Stock incentive plans
Shares of common stock issued resulting from vesting	239,000
RSUs
Number of Shares
Non-vested at beginning of period (in shares)	633,000
Granted (in shares)	847,000
Vested (in shares)	(347,000)
Canceled/forfeited (in shares)	(57,000)
Non-vested at end of period (in shares)	1,076,000
Weighted Average Grant Date Fair Value
Non-vested at beginning of period (in dollars per share) | $ / shares	$ 22.04
Granted (in dollars per share) | $ / shares	11.07
Vested (in dollars per share) | $ / shares	18.66
Canceled/forfeited (in dollars per share) | $ / shares	17.97
Non-vested at end of period (in dollars per share) | $ / shares	$ 14.71
Service-based restricted stock unit awards | Executive And Non Executive Member | Maximum
Stock incentive plans
Vesting period	3 years
Service-based restricted stock unit awards | Non-employee directors and existing members
Stock incentive plans
Vesting period	1 year
Service-based restricted stock unit awards | Non-employee directors and new members
Stock incentive plans
Vesting percentage when the individual completes 12 months of continuous service	33.30%
Vesting period	3 years
Performance-based restricted stock unit awards
Stock incentive plans
Vesting period	3 years
Performance-based restricted stock unit awards | Minimum
Stock incentive plans
Performance objective threshold (as a percent)	50.00%
Performance-based restricted stock unit awards | Maximum
Stock incentive plans
Performance objective threshold (as a percent)	150.00%
2018 performance-based RSUs
Stock incentive plans
Performance objective achieved (as a percent)	100.50%
2019 and 2020 performance-based RSUs
Stock incentive plans
Vesting period	3 years
2019 performance-based RSUs
Stock incentive plans
Number of awards excluded from granted and non-vested as the performance targets have not yet been defined	36,000
Number of Shares
Granted (in shares)	36,000
Weighted Average Grant Date Fair Value
Granted (in dollars per share) | $ / shares	$ 12.41
2020 performance-based RSUs
Stock incentive plans
Number of awards excluded from granted and non-vested as the performance targets have not yet been defined	178,000