ARO Trust (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 01, 2020	Dec. 31, 2019	Mar. 01, 2019
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	$ 184.0		$ 167.6
Trading Securities, Equity	214.5		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	21.4		15.8
Trading Securities, Equity	21.4		15.8
U.S. Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	59.5		55.3
Trading Securities, Equity	86.6		83.3
International Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	34.2		31.8
Trading Securities, Equity	35.7		35.4
Municipal Bond Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	68.9		64.7
Trading Securities, Equity	$ 70.8		$ 66.1