ARO Trust (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	$ 181.7	$ 178.0
Debt and Equity Securities, Fair Value	249.6	260.5
External trust [Member]
Debt Securities, Available-for-sale [Line Items]
Funding obligation	16.0
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	9.7	5.9
Debt and Equity Securities, Fair Value	9.7	5.9
U.S. Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	52.6	52.7
Debt and Equity Securities, Fair Value	114.4	121.4
International Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	31.7	31.7
Debt and Equity Securities, Fair Value	40.5	43.2
Municipal Bond Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	87.7	87.7
Debt and Equity Securities, Fair Value	$ 85.0	$ 90.0

Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

No definition available.

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- Definition

Cost of investment in debt investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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