Commitments and Contingencies Sherco (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
NSP Minnesota
Public Utilities, General Disclosures [Line Items]
Percentage of Fault			48.00%
Customer refund of previously recovered purchased power costs	$ 20
General Electric (GE) [Domain]
Public Utilities, General Disclosures [Line Items]
Percentage of Fault		52.00%