Fair Value of Financial Assets and Liabilities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
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	$ (200,000)		$ (200,000)
Commodity Derivatives [Abstract]
Number of commodity derivatives	0		0		0
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss related to cash flow hedges at beginning of period	(1,418,000)	(1,589,000)	(1,504,000)	(1,675,000)
After-tax net realized losses on derivative transactions reclassified into earnings	43,000	42,000	129,000	128,000
Accumulated other comprehensive loss related to cash flow hedges at end of period	(1,375,000)	(1,547,000)	(1,375,000)	(1,547,000)
Carrying Amount [Member]
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	1,103,625,000		1,103,625,000		993,314,000
Fair Value [Member]
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	1,341,654,000		1,341,654,000		1,176,020,000
Cash Flow Hedges [Member] | Interest Rate [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, 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	200,000	200,000
Fair Value Measured on a Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0		0		0
Total liabilities	$ 0		$ 0		$ 0