Commitments and Contingencies, Legal Contingencies (Details) - Gas Trading Litigation	1 Months Ended
	Nov. 30, 2017	Dec. 31, 2017	Dec. 31, 2009
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number		6	13
Loss Contingency, Subset of Cases within Multi-District Litigation, Number		2
NSP-Wisconsin
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number			2
Summary Judgment Granted Against Plaintiff [Member]
Loss Contingencies [Line Items]
Loss Contingency, Number of Plaintiffs	2
Remaining in the Litigation [Member]
Loss Contingencies [Line Items]
Loss Contingency, Number of Plaintiffs	3