Commitments and Contingencies Commitments and Contingencies - Legal Contingencies (Details) - Gas Trading Litigation	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2009
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number	7	13
Loss Contingency, Claims Settled, Number	5
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