Share-based compensation - Common Stock Granted to Nonemployee Directors (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share-based compensation
Income tax benefit	$ 0.5	$ 0.4	$ 1.6	$ 1.5
Common stock
Share-based compensation
Shares granted (in shares)	0	965	40,450	35,720
Fair value	$ 0.0	$ 0.0	$ 1.5	$ 1.5
Income tax benefit	$ 0.0	$ 0.0	$ 0.4	$ 0.4