Stock-based compensation - Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	8 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	May 15, 2016
Number of shares (in thousands)
Outstanding beginning balance (shares)	0	208,000
Granted (shares)	208,000	109,000
Exercised (shares)	0	(7,000)
Forfeited (shares)	0	(7,000)
Canceled (shares)	0	0
Outstanding ending balance (shares)	208,000	303,000
Stock options exercisable (shares)		5,000
Weighted-average exercise price (per share)
Weighted average exercise price (per share) granted	$ 28.03	$ 53.11
Exercised (dollars per share)	0.00	27.38
Forfeited (dollars per share)	0.00	31.97
Canceled (dollars per share)	0.00	0.00
Weighted average exercise price (per share) outstanding	$ 28.03	36.72	$ 0.00
Weighted average exercise price (per share) exercisable		$ 27.90
Weighted-average remaining contractual term (years)	9 years 5 months	8 years 8 months
Exercisable, weighted-average remaining contractual term (years)		8 years 5 months
Aggregate intrinsic value	$ 5,573	$ 10,022
Stock options exercisable, aggregate intrinsic value		$ 213