ARO Trust (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	$ 190.9	$ 188.4
Debt and Equity Securities, Fair Value	242.3	229.7
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	18.9	16.4
Debt and Equity Securities, Fair Value	18.9	16.4
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	102.6	96.1
International Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	31.7	31.6
Debt and Equity Securities, Fair Value	37.4	35.2
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	$ 83.4	$ 82.0