Share-based compensation plans - Option Activity (Details)	9 Months Ended
	Sep. 30, 2020 $ / shares shares
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares | shares	1,191,100
Granted, shares | shares	0
Forfeited, shares | shares	0
Exercised, shares | shares	0
Ending balance, shares | shares	1,191,100
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) | $ / shares	$ 11.52
Granted (dollars per share) | $ / shares	0
Forfeited (dollars per share) | $ / shares	0
Exercised (dollars per share) | $ / shares	0
Ending balance (dollars per share) | $ / shares	$ 11.52
Time Based Vesting Option
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, shares | shares	127,000.0
Granted, shares | shares	80,800
Forfeited, shares | shares	0
Exercised, shares | shares	0
Ending balance, shares | shares	207,800
Weighted Average Grant Date Fair Value Per Share
Beginning balance (dollars per share) | $ / shares	$ 21.56
Granted (dollars per share) | $ / shares	21.10
Forfeited (dollars per share) | $ / shares	0
Exercised (dollars per share) | $ / shares	0
Ending balance (dollars per share) | $ / shares	$ 21.38