Schedule of Power Cost Adjustment Mechanism (Details) - Subsidiaries [Member] $ in Millions		3 Months Ended		12 Months Ended
	Dec. 31, 2019 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability, Interest		$ 0.3	$ 0.5
Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability, Amount		11.4	25.1	$ 67.2
Customer's share [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability, Amount		$ 0.0	$ 4.0	36.0
Customer's share [Member] | Range 1 [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0
Customer's share [Member] | Range 1 [Member] | Over-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0
Customer's share [Member] | Range 2 [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.50
Customer's share [Member] | Range 2 [Member] | Over-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.65
Customer's share [Member] | Range 3 [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.90
Customer's share [Member] | Range 3 [Member] | Over-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.90
Companys share [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability, Amount				31.2
Companys share [Member] | Range 1 [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		1
Companys share [Member] | Range 1 [Member] | Over-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		1
Companys share [Member] | Range 2 [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.50
Companys share [Member] | Range 2 [Member] | Over-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.35
Companys share [Member] | Range 3 [Member] | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.10
Companys share [Member] | Range 3 [Member] | Over-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability		0.10
Customer's share plus interest | Under-collection [Member]
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability, Amount	$ 41.7			37.0	$ 4.7
Customer's share plus interest | Maximum Power
Regulatory Assets and Liabiliaties [Line Items]
Annual Power Cost Variability, Amount				$ 20.0