Income Taxes - Valuation Allowance for Deferred Tax Assets (Details) - Valuation allowance for deferred tax assets - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Valuation allowance for deferred tax assets at beginning of the period	$ 67.9	$ 72.5	$ 47.9
Revaluation or additions due to acquisitions or mergers	63.3	0.0	0.0
Change due to U.S. Tax Reform	0.0	0.0	23.4
Charged to tax expense	8.9	(5.4)	(4.2)
Charged to other accounts	1.1	0.1	(1.3)
Deductions	0.1	0.7	6.7
Valuation allowance for deferred tax assets at end of the period	$ 141.3	$ 67.9	$ 72.5