ARO Trust (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Annual funding obligation	$ 36.4
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	134.8	$ 116.0
Trading Securities, Equity	156.8	134.6
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	16.4	12.6
Trading Securities, Equity	16.4	12.6
U.S. Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	46.4	35.9
Trading Securities, Equity	66.7	50.5
International Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	21.9	20.7
Trading Securities, Equity	24.5	24.6
Municipal Bond Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	50.1	46.8
Trading Securities, Equity	$ 49.2	$ 46.9