Commitments and Contingencies Commitments and Contingencies - Legal Contingencies (Details) - Gas Trading Litigation	Sep. 30, 2017	Dec. 31, 2009
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number	6	13
Loss Contingency, Multi-District Litigation Claims, Number	1
Loss Contingency, Subset of Cases within Multi-District Litigation, Number	2
NSP-Wisconsin
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number		2