Stock incentive plans - Stock option awards (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Aggregate Intrinsic Value
Cash proceeds from exercise of stock option	$ 9,979	$ 9,244	$ 2,984
Stock options
Stock incentive plans
Vesting period	4 years
Number of Options
Outstanding at beginning of year (in shares)	1,283,000
Exercised (in shares)	(972,000)	(1,776,000)	(532,000)
Canceled/forfeited (in shares)	(7,000)
Outstanding and exercisable at end of year (in shares)	304,000	1,283,000
Weighted Average Exercise Price
Outstanding at beginning of year (in dollars per share)	$ 9.58
Exercised (in dollars per share)	10.26
Canceled/forfeited (in dollars per share)	5.99
Outstanding and exercisable at end of year (in dollars per share)	$ 7.49	$ 9.58
Weighted-Average Remaining Contract Life (years)
Outstanding and exercisable at end of year	3 years 9 months 18 days	3 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding and exercisable at end of year	$ 3,970	$ 16,573
Total intrinsic value of stock options exercised (in dollars)	$ 14,935	$ 20,551	$ 1,675
Stock options | Vesting in 12 months
Stock incentive plans
Vesting period	12 months
Vesting percentage	25.00%
Stock options | Vesting monthly 12 months after grant date
Stock incentive plans
Vesting period	4 years
Vesting percentage	75.00%