Income Taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Taxes
Unrecognized tax benefits at the beginning of year	$ 14.2	$ 13.7	$ 11.4
Gross increases - tax positions in prior period	0.1		0.7
Gross decreases - tax positions in prior period	(0.4)	(0.7)
Gross increases - tax positions in current period	1.0	1.2	1.6
Settlements	(2.7)
Unrecognized tax benefits	12.2	14.2	13.7
Accrued gross interest and penalties	$ 0.4	$ 1.4	$ 1.2