ARO Trust (Details) - USD ($) $ in Millions	Mar. 31, 2020	Mar. 01, 2020	Dec. 31, 2019	Mar. 01, 2019
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	$ 180.7		$ 167.6
Trading Securities, Equity	188.2		200.6
External trust [Member]
Debt Securities, Available-for-sale [Line Items]
Other Commitment, Due in Next Twelve Months		$ 16.0	35.9	$ 23.8
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	22.4		15.8
Trading Securities, Equity	22.4		15.8
U.S. Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	63.3		55.3
Trading Securities, Equity	74.5		83.3
International Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	36.8		31.8
Trading Securities, Equity	32.3		35.4
Municipal Bond Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	58.2		64.7
Trading Securities, Equity	$ 59.0		$ 66.1