ARO Trust (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Debt and Equity Securities, Cost	$ 187.3	$ 178.0
Debt and Equity Securities, Fair Value	216.2	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	15.3	5.9
Debt and Equity Securities, Fair Value	15.3	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	90.1	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	31.0	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	$ 79.8	$ 90.0