Fair Value Measurements (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 0	$ 0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	0	0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount	0	0
Quoted Prices In Active Markets for Identical Assets (Level 1)
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	0	0
Significant Other Oberservable Inputs (Level 2)
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	(5,251,400,000)	(4,785,500,000)
Significant Unobservable Inputs (Level 3)
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	0	0
Carrying Amount
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	(4,065,000,000)	(4,014,400,000)
Fair Value
Additional Fair Value Elements [Abstract]
Long-term debt. including current portion	(5,251,400,000)	(4,785,500,000)
ARO Trust investments \| Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	200,600,000	149,500,000
ARO Trust investments \| Significant Other Oberservable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	0	0
ARO Trust investments \| Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	0	0
ARO Trust investments \| Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	200,600,000	149,500,000
ARO Trust investments \| Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
ARO Trust investments	$ 200,600,000	$ 149,500,000

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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No definition available.

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

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Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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