Stock incentive plans - Restricted stock unit awards (Details) - $ / shares	1 Months Ended	9 Months Ended
	Feb. 28, 2019	Sep. 30, 2019
Stock incentive plans
Shares of common stock issued resulting from vesting		1,481,000
RSUs
Number of Shares
Non-vested at beginning of period (in shares)		3,119,000
Granted (in shares)		637,000
Vested (in shares)		(2,896,000)
Canceled/forfeited (in shares)		(60,000)
Non-vested at end of period (in shares)		800,000
Weighted Average Grant Date Fair Value
Non-vested at beginning of period (in dollars per share)		$ 8.60
Granted (in dollars per share)		22.15
Vested (in dollars per share)		7.56
Canceled/forfeited (in dollars per share)		23.71
Non-vested at end of period (in dollars per share)		$ 21.97
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
Number of Shares
Granted (in shares)	29,000
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 11.94
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%