ARO Trust (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	$ 178.0	$ 174.8
Debt and Equity Securities, Fair Value	260.5	235.5
External ARO trust [Member]
Debt Securities, Available-for-sale [Line Items]
Annual funding obligation	16.0
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	5.9	5.8
Debt and Equity Securities, Fair Value	5.9	5.8
U.S. Equity Funds
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	52.7	59.9
Debt and Equity Securities, Fair Value	121.4	108.0
International Equity Funds
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	31.7	34.2
Debt and Equity Securities, Fair Value	43.2	43.7
Municipal Bond Funds
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	87.7	74.9
Debt and Equity Securities, Fair Value	$ 90.0	$ 78.0

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.

No definition available.

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- Definition

Amount of commitment classified as other to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

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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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