Income Taxes - Uncertain tax positions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Y	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Income Taxes
Unrecognized tax benefits at the beginning of year	$ 28.0	$ 30.9	$ 31.5
Gross increases - tax positions in prior period			3.8
Gross decreases - tax positions in prior period	(12.8)	(2.9)	(2.6)
Gross increases - tax positions in current period			14.2
Settlements	(3.8)		(1.2)
Gross change related to Tax Reform			(14.8)
Unrecognized tax benefits at the end of year	11.4	28.0	30.9
Accrued gross interest and penalties	1.0	$ 1.9	$ 1.4
Unrecognized tax benefits reasonably possible adjustment in the next twelve months	$ 0.0
Number of tax years open for examination | Y	0