Goodwill and Intangible Assets - Changes in Carrying Value (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Finite-lived Intangible Assets [Roll Forward]
Accumulated Amortization, Beginning Balance			$ (545.9)
Amortization expense	$ (30.9)	$ (31.0)	(61.8)	$ (61.9)
Retirements, net			(1.0)
Accumulated Amortization, Ending Balance	(607.1)		(607.1)
Net Carrying Amount, Ending Balance	1,187.1		1,187.1
Louisiana
Finite-lived Intangible Assets [Roll Forward]
Retirements, gross			(1.6)
Customer relationships
Finite-lived Intangible Assets [Roll Forward]
Gross Carrying Amount, Beginning Balance			1,795.8
Accumulated Amortization, Beginning Balance			(545.9)
Net Carrying Amount, Beginning Balance			1,249.9
Amortization expense			(61.8)
Retirements, accumulated amortization			0.6
Gross Carrying Amount, Ending Balance	1,794.2		1,794.2
Accumulated Amortization, Ending Balance	(607.1)		(607.1)
Net Carrying Amount, Ending Balance	$ 1,187.1		$ 1,187.1