Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Valuation and qualifying accounts
Balance at beginning of period	$ 140.0	$ 149.2	$ 143.2
Expense	102.8	85.8	94.7
Deferral	55.3	11.4	(5.5)
Net write-offs	(77.9)	(106.4)	(83.2)
Sale of business	(0.1)	0.0	0.0
Balance at end of period	$ 220.1	$ 140.0	$ 149.2