Commitments and Contingencies Commitments and Contingencies - Legal Contingencies (Details) - Gas Trading Litigation [Member]	12 Months Ended		84 Months Ended
	Dec. 31, 2011	Dec. 31, 2009	Dec. 31, 2009
Loss Contingencies [Line Items]
Loss Contingency, Number of Plaintiffs	7		13
Loss Contingency, Claims Settled, Number		5
Loss Contingency, Claims Dismissed, Number	6	1
NSP-Wisconsin
Loss Contingencies [Line Items]
Loss Contingency, Number of Plaintiffs			2