Share-based compensation - Common Stock Granted to Nonemployee Directors (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Share-based compensation
Income tax benefit	$ 0.8	$ 0.8	$ 1.1	$ 1.1
Common stock
Share-based compensation
Shares granted (in shares)	38,941	34,755	40,450	34,755
Fair value	$ 1.4	$ 1.4	$ 1.5	$ 1.4
Income tax benefit	$ 0.4	$ 0.4	$ 0.4	$ 0.4