Regulatory Assets and Liabilities Regulatory assets and liabilities Phantom (Details)	12 Months Ended
	Dec. 31, 2019
Benson purchase power agreement termination and asset purchase
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	10 years
Laurentian biomass PPA termination
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	5 years
PI extended power update
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	16 years
Conservation Programs | Minimum [Member]
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	1 year
Conservation Programs | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	2 years
Nuclear Refueling Outage Costs | Minimum [Member]
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	1 year
Nuclear Refueling Outage Costs | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	2 years
Deferred Purchased Natural Gas and Electric Energy Costs | Minimum [Member]
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	1 year
Deferred Purchased Natural Gas and Electric Energy Costs | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	3 years
Sales True-Up and Revenue Decoupling | Minimum [Member]
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	1 year
Sales True-Up and Revenue Decoupling | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	2 years
Renewable Resources and Environmental Initiatives | Minimum [Member]
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	1 year
Renewable Resources and Environmental Initiatives | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Amortization Period	2 years