Accounting for Derivative Instruments and Hedging Activities Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Subsidiaries [Member]
Derivative [Line Items]
Hedging strategy number of years extended	3 years
Natural Gas Portfolio [Member]
Derivative [Line Items]
Posted Collateral	$ 12.0
External Credit Rating, Investment Grade [Member]
Derivative [Line Items]
Derivative, Credit Risk Exposure, Percentage	99.20%
External Credit Rating, Non Investment Grade [Member]
Derivative [Line Items]
Derivative, Credit Risk Exposure, Percentage	0.80%
Credit Rating [Member]
Derivative [Line Items]
Posted Collateral	$ 3.2
Credit Rating [Member] | Natural Gas Portfolio [Member]
Derivative [Line Items]
Posted Collateral	$ 1.0