Fair Value of Financial Assets and Liabilities (Details) (USD $)											6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Carrying Amount [Member]	Dec. 31, 2011 Carrying Amount [Member]	Jun. 30, 2012 Fair Value [Member]	Dec. 31, 2011 Fair Value [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Level 1 [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Level 2 [Member]	Dec. 31, 2011 Fair Value Measured on a Recurring Basis [Member] Level 3 [Member]	Jun. 30, 2012 Cash Flow Hedges [Member] Interest Rate [Member]	Jun. 30, 2011 Cash Flow Hedges [Member] Interest Rate [Member]	Jun. 30, 2011 Other Derivative Instruments [Member] Natural Gas Commodity [Member]	Jun. 30, 2012 Other Derivative Instruments [Member] Natural Gas Commodity [Member]	Jun. 30, 2011 Other Derivative Instruments [Member] Natural Gas Commodity [Member]	Jun. 30, 2012 Interest Rate Swap [Member]	Jun. 30, 2012 Natural Gas Commodity [Member] MMBTU		Dec. 31, 2011 Natural Gas Commodity [Member] MMBTU
Interest Rate Derivatives [Abstract]
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months																$ (100,000)
Gross Notional Amounts of Commodity Forwards and Options [Abstract]
Notional amount																	177,000	[1],[2]	1,393,000	[1],[2]
Impact of Derivative Activity on OCI, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss											(100,000)	(100,000)
Fair value gains (losses) recognized during the period in regulatory (assets) and liabilities													(200,000)	(400,000)	(500,000)
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)														2,900,000	2,000,000
Financial Impact of Qualifying Fair Value Hedges [Abstract]
Derivative instruments designated as fair value hedges	0	0
Other derivative instruments [Abstract]
Natural gas commodity							2,514,000	418,000	2,096,000	0
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion			$ 369,391,000	$ 369,369,000	$ 482,869,000	$ 474,356,000

[1] Amounts are not reflective of net positions in the underlying commodities.

[2] Notional amounts for options are also included on a gross basis, but are weighted for the probability of exercise.