Summary of Significant Accounting Policies - Schedule of Change in Conditional AROs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of year	$ 51.4	$ 46.0	$ 44.0
(Reductions)/ additions and other, net	(2.7)	2.6	(0.5)
Accretion	2.7	2.8	2.5
Balance at end of year	$ 51.4	$ 51.4	$ 46.0