Commitments and Contingencies, Legal Contingencies (Details)	1 Months Ended
	Dec. 31, 2015	Jun. 30, 2018	Dec. 31, 2009
Gas Trading Litigation
Legal Contingencies [Abstract]
Loss Contingency, Pending Claims, Number		6	13
Loss Contingency, Subset of Cases within Multi-District Litigation, Number		2
NSP-Wisconsin | Gas Trading Litigation
Legal Contingencies [Abstract]
Loss Contingency, Pending Claims, Number			2
Minimum | PSCo | Line Extension Disputes
Legal Contingencies [Abstract]
Loss Contingency, Number of Plaintiffs	50