Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Prepayments and other	$ 5,056		$ 4,394
Other current liabilities	9,376		10,955
Fair Value Measured on a Recurring Basis [Member] | Other Current Assets [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	580	[1]
Fair Value Measured on a Recurring Basis [Member] | Other Current Liabilities [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability			11	[2]
Fair Value Measured on a Recurring Basis [Member] | Level 1 | Other Current Assets [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0
Fair Value Measured on a Recurring Basis [Member] | Level 1 | Other Current Liabilities [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability			0
Fair Value Measured on a Recurring Basis [Member] | Level 2 | Other Current Assets [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	580
Fair Value Measured on a Recurring Basis [Member] | Level 2 | Other Current Liabilities [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability			11
Fair Value Measured on a Recurring Basis [Member] | Level 3 | Other Current Assets [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0
Fair Value Measured on a Recurring Basis [Member] | Level 3 | Other Current Liabilities [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability			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]
Prepayments and other	5,100
Other current liabilities			11,000
Fair Value Measured on a Recurring Basis [Member] | Fair Value Total [Member] | Other Current Assets [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	580
Fair Value Measured on a Recurring Basis [Member] | Fair Value Total [Member] | Other Current Liabilities [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability			11
Fair Value Measured on a Recurring Basis [Member] | Counterparty Netting [Member] | Other Current Assets [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	0	[3]
Fair Value Measured on a Recurring Basis [Member] | Counterparty Netting [Member] | Other Current Liabilities [Member] | Natural Gas Commodity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability, Fair Value, Gross Liability			$ 0	[3]

[1] Included in other current assets balance of $5.1 million at Dec. 31, 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 Dec. 31, 2013 and 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.