Summary of Significant Accounting Policies (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of year	$ 34.7	$ 34.0	$ 34.3
Additions charged to expense	19.9	27.8	34.0
Accounts written-off	(16.1)	(27.1)	(34.3)
Balance at end of year	$ 38.5	$ 34.7	$ 34.0