Regulatory Assets and Liabilities Regulatory Assets and Liabilities Phantom (Details)	12 Months Ended
	Dec. 31, 2019
Depreciation differences | Minimum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	1 year
Depreciation differences | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	12 years
Benson Biomass PPA termination and asset purchase
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	10 years
Laurentian biomass PPA termination
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	5 years
PI extended power update
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	16 years
Conservation programs | Minimum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	1 year
Conservation programs | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	2 years
Nuclear refueling outage costs | Minimum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	1 year
Nuclear refueling outage costs | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	2 years
Deferred purchased natural gas and electric energy costs | Minimum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	1 year
Deferred purchased natural gas and electric energy costs | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	3 years
Renewable resources and environmental initiatives | Minimum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	1 year
Renewable resources and environmental initiatives | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	2 years
Sales true-up and revenue decoupling | Minimum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	1 year
Sales true-up and revenue decoupling | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	2 years
Gas pipeline inspection and remediation costs | Minimum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	1 year
Gas pipeline inspection and remediation costs | Maximum
Regulatory Assets [Line Items]
Regulatory Asset, Remaining Amortization Period	2 years