Income Taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Taxes
Unrecognized tax benefits at the beginning of year	$ 13.7	$ 11.4	$ 28.0
Gross increases - tax positions in prior period		0.7
Gross decreases - tax positions in prior period	(0.7)		(12.8)
Gross increases - tax positions in current period	1.2	1.6
Settlements			(3.8)
Unrecognized tax benefits	14.2	13.7	11.4
Unrecognized tax benefits, if recognized, would affect effective tax rate	14.2
Accrued gross interest and penalties	1.4	$ 1.2	$ 1.0
Unrecognized tax benefits that could reverse in the next twelve months	2.7
Interest accrued on uncertain income tax positions that could reverse in the next twelve months	$ 1.4