ARO Trust (Details) - USD ($) $ in Millions	Dec. 31, 2020	Mar. 01, 2020	Dec. 31, 2019	Mar. 01, 2019
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	$ 174.8		$ 167.6
Trading Securities, Equity	235.5		200.6
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	5.8		15.8
Trading Securities, Equity	5.8		15.8
U.S. Equity Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	59.9		55.3
Trading Securities, Equity	108.0		83.3
International Equity Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	34.2		31.8
Trading Securities, Equity	43.7		35.4
Municipal Bond Funds
Debt Securities, Available-for-sale [Line Items]
Trading Securities, Equity, Cost	74.9		64.7
Trading Securities, Equity	$ 78.0		$ 66.1
External ARO trust [Member]
Debt Securities, Available-for-sale [Line Items]
Annual funding obligation		$ 16.0
External ARO trust [Member] | Rate Case filing [Member]
Debt Securities, Available-for-sale [Line Items]
Annual funding obligation				$ 35.9
External ARO trust [Member] | Rate Case settlement [Member]
Debt Securities, Available-for-sale [Line Items]
Annual funding obligation				$ 23.8