ARO Trust (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	$ 167.6	$ 140.1
Trading Securities, Equity	200.6	149.5
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	15.8	21.7
Trading Securities, Equity	15.8	21.7
U.S. Equity Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	55.3	46.4
Trading Securities, Equity	83.3	56.8
International Equity Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	31.8	21.9
Trading Securities, Equity	35.4	21.4
Municipal Bond Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	64.7	50.1
Trading Securities, Equity	66.1	$ 49.6
External ARO trust [Member]
Investments, Debt and Equity Securities [Abstract]
Annual funding obligation	$ 35.9