ARO Trust (Details) - USD ($) $ in Millions	Mar. 31, 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	152.2	$ 140.1
Trading Securities, Equity	172.8	149.5
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	27.8	21.7
Trading Securities, Equity	27.8	21.7
U.S. Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	50.5	46.4
Trading Securities, Equity	68.9	56.8
International Equity Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	23.8	21.9
Trading Securities, Equity	25.6	21.4
Municipal Bond Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	50.1	50.1
Trading Securities, Equity	$ 50.5	$ 49.6