Share-based compensation plans - Option Activity (Details)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares | shares	879,800
Granted, shares | shares	417,600
Forfeited, shares | shares	0
Exercised, shares | shares	0
Ending balance, shares | shares	1,297,400
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) | $ / shares	$ 8.04
Granted (dollars per share) | $ / shares	16.73
Forfeited (dollars per share) | $ / shares	0.00
Exercised (dollars per share) | $ / shares	0.00
Ending balance (dollars per share) | $ / shares	$ 10.83
Time Based Vesting Option
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares | shares	40,600
Granted, shares | shares	0
Forfeited, shares | shares	0
Exercised, shares | shares	0
Ending balance, shares | shares	40,600
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) | $ / shares	$ 8.66
Granted (dollars per share) | $ / shares	0.00
Forfeited (dollars per share) | $ / shares	0.00
Exercised (dollars per share) | $ / shares	0.00
Ending balance (dollars per share) | $ / shares	$ 8.66