Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Prepayments and other		$ 3,966	$ 6,918
Other Liabilities, Current		22,170	19,923
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		3
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0
Fair Value Measured on a Recurring Basis | Fair Value Total
Derivatives, Fair Value [Line Items]
Prepayments and other			6,900
Other Liabilities, Current		22,200
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		3
Fair Value Measured on a Recurring Basis | Netting | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[2]	(3)
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0	52	[1]
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[1]	45
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Other Current Liabilities [Member] | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		45
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0	0
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities [Member] | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		3	52
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		48
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities [Member] | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		48
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0	0
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities [Member] | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		3	52
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		48
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities [Member] | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		48
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Netting | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[2]	(3)	$ 0
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[2]	(3)
Other Derivative Instruments | Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities [Member] | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	[2]	$ (3)

[1] Included in other current liabilities balance of $22.2 million at June 30, 2015 and prepayments and other assets balance of $6.9 million at Dec. 31, 2014, in the consolidated balance sheets.

[2] 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. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.