Power Cost Adjustment Clause Filing (Details) - Subsidiaries [Member] - USD ($) $ in Millions			12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Under-collection
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount			$ 75.4	$ 67.2
Under-collection | Companys share
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount				31.2
Under-collection | Customer's share
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount			$ 43.3	36.0
Under-collection | Customer's share plus interest [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount	$ 41.7	$ 4.7		37.0
Maximum Power | Customer's share plus interest [Member]
Regulatory Assets [Line Items]
Annual Power Cost Variability, Amount				$ 20.0