Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	$ 254.4	$ 229.7
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	(5,340.9)	(5,361.7)
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	(5,272.5)	(5,280.3)
Estimate of Fair Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Long-term debt, including current portion	(5,340.9)	(5,361.7)
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	254.4	229.7
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	254.4	229.7
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	$ 254.4	$ 229.7