Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other current liabilities	$ 9,627		$ 10,955
Prepayments and other	2,889		4,394
Fair Value Measured on a Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Natural gas commodity	346	[1]
Current derivative liabilities [Abstract]
Natural gas commodity			11	[2]
Fair Value Measured on a Recurring Basis [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Natural gas commodity	0
Current derivative liabilities [Abstract]
Natural gas commodity			0
Fair Value Measured on a Recurring Basis [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Natural gas commodity	346
Current derivative liabilities [Abstract]
Natural gas commodity			11
Fair Value Measured on a Recurring Basis [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Natural gas commodity	0
Current derivative liabilities [Abstract]
Natural gas commodity			0
Fair Value Measured on a Recurring Basis [Member] | Fair Value Total [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other current liabilities			11,000
Prepayments and other	2,900
Natural gas commodity	346
Current derivative liabilities [Abstract]
Natural gas commodity			11
Fair Value Measured on a Recurring Basis [Member] | Counterparty Netting [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Natural gas commodity	0
Current derivative liabilities [Abstract]
Natural gas commodity			$ 0	[3]

[1] Included in other current assets balance of $2.9 million at Sept. 30, 2013 in the consolidated balance sheets.

[2] Included in other current liabilities balance of $11.0 million at Dec. 31, 2012 in the consolidated balance sheets.

[3] NSP-Wisconsin nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2013 and Dec. 31, 2012. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.