Revenue from Contracts with Customers - Provision for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in the allowance for doubtful accounts
Balance at beginning of period	$ 7.5
Provision charged to expense	16.8	$ 16.9	$ 20.2
Balance at end of period	8.6	7.5
Trade accounts receivable
Change in the allowance for doubtful accounts
Balance at beginning of period	7.5	7.5
Provision charged to expense	16.4	16.9
Accounts written off, net of recoveries	(15.3)	(16.9)
Balance at end of period	8.6	7.5	$ 7.5
Non-trade accounts receivable
Change in the allowance for doubtful accounts
Provision charged to expense	$ 0.4	$ 0.0