ARO Trust (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	$ 194.8	$ 188.4
Debt and Equity Securities, Fair Value	254.4	229.7
External trust [Member]
Marketable Securities [Line Items]
Funding obligation	16.0
Money Market Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	22.8	16.4
Debt and Equity Securities, Fair Value	22.8	16.4
U.S. Equity Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	52.6	52.7
Debt and Equity Securities, Fair Value	110.8	96.1
International Equity Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	31.7	31.6
Debt and Equity Securities, Fair Value	38.0	35.2
Municipal Bond Funds [Member]
Marketable Securities [Line Items]
Debt and Equity Securities, Cost	87.7	87.7
Debt and Equity Securities, Fair Value	$ 82.8	$ 82.0

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

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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