Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Valuation and qualifying accounts
Balance at beginning of period	$ 149.2	$ 143.2	$ 108.0
Expense	85.8	94.7	96.7
Deferral	11.4	(5.5)	16.4
Net write-offs	(106.4)	(83.2)	(77.9)
Balance at end of period	$ 140.0	$ 149.2	$ 143.2