Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	$ 247.8	$ 235.5
Fair Value, Inputs, Level 1 [Member]
Additional Fair Value Elements [Abstract]
Long-term debt, including current portion	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Additional Fair Value Elements [Abstract]
Long-term debt, including current portion	(6,700.8)	(6,949.0)
Fair Value, Inputs, Level 3 [Member]
Additional Fair Value Elements [Abstract]
Long-term debt, including current portion	0.0	0.0
Reported Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Long-term debt, including current portion	(5,226.9)	(5,239.8)
Estimate of Fair Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Long-term debt, including current portion	(6,700.8)	(6,949.0)
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	247.8	235.5
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	0.0	0.0
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	0.0	0.0
Fair Value, Recurring [Member] | Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	247.8	235.5
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	$ 247.8	$ 235.5