Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Prepayments and other	$ 6,918		$ 5,056
Other current liabilities	19,923		22,521
Fair Value Measured on a Recurring Basis | 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	52	[1]	580	[2]
Fair Value Measured on a Recurring Basis | 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		0
Fair Value Measured on a Recurring Basis | 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	52		580
Fair Value Measured on a Recurring Basis | 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		0
Fair Value Measured on a Recurring Basis | Fair Value Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Prepayments and other	6,900
Other current liabilities			5,100
Fair Value Measured on a Recurring Basis | Fair Value Total | 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	52		580
Fair Value Measured on a Recurring Basis | 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]	$ 0	[3]

[1] Included in other current assets balance of $6.9 million at Dec. 31, 2014 in the consolidated balance sheets.

[2] Included in other current assets balance of $5.1 million at Dec. 31, 2013 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, 2014 and 2013. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.