Revenue from Contracts with Customers - Provision for Doubtful Accounts (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenue from Contracts with Customers
Threshold period past due for trade accounts receivable for disconnecting service to customers	60 days
Change in the allowance for doubtful accounts
Balance at beginning of year	$ 7.5	$ 7.5
Provision charged to expense	5.3	3.5
Accounts written off, net of recoveries	(3.8)	(4.0)
Balance at end of year	9.0	$ 7.0
Allowance for doubtful accounts for non-trade accounts receivable	$ 2.0