Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	$ 282.9	$ 268.9
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	(5,267.2)	(5,438.4)
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	(5,249.0)	(5,261.2)
Estimate of Fair Value Measurement [Member]
Additional Fair Value Elements [Abstract]
Long-term debt, including current portion	(5,267.2)	(5,438.4)
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	282.9	268.9
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	282.9	268.9
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	$ 282.9	$ 268.9