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