ARO Trust (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Annual funding obligation	$ 35.9
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	162.7	$ 140.1
Trading Securities, Equity	187.3	149.5
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	23.3	21.7
Trading Securities, Equity	23.3	21.7
U.S. Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	54.7	46.4
Trading Securities, Equity	76.2	56.8
International Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	25.4	21.9
Trading Securities, Equity	27.0	21.4
Municipal Bond Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	59.3	50.1
Trading Securities, Equity	$ 60.8	$ 49.6