Share-based compensation - Common Stock Granted to Nonemployee Directors (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Share-based compensation
Income tax benefit	$ 0.2	$ 0.8	$ 0.3	$ 1.1
Common stock
Share-based compensation
Shares granted (in shares)	0	38,941	0	40,450
Fair value	$ 0.0	$ 1.4	$ 0.0	$ 1.5
Income tax benefit	$ 0.0	$ 0.4	$ 0.0	$ 0.4