Supplementary Financial Statement Information - Allowance for Credit Losses (Details2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Credit Losses
Balance at beginning of year	$ 6.6	$ 6.8	$ 7.8
Increase (decrease) in allowance	(1.6)	0.3	(1.0)
Receivables written off	(0.7)	(0.5)	0.0
Balance at end of year	$ 4.3	$ 6.6	$ 6.8