Fair Value of Financial Assets and Liabilities (Details) (USD $)							3 Months Ended		6 Months Ended
	Jun. 30, 2012	Dec. 31, 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]	Jun. 30, 2012 Cash Flow Hedges [Member] Interest Rate [Member]	Jun. 30, 2011 Cash Flow Hedges [Member] Interest Rate [Member]	Jun. 30, 2012 Cash Flow Hedges [Member] Interest Rate [Member]	Jun. 30, 2011 Cash Flow Hedges [Member] Interest Rate [Member]	Jun. 30, 2012 Fair Value Measured on a Recurring Basis [Member]	Dec. 31, 2011 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
Total liabilities											0	0
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)
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)	(100,000)	(100,000)
Commodity Derivatives [Abstract]
Number of commodity derivatives	0	0
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion			$ 1,103,566,000	$ 993,314,000	$ 1,306,907,000	$ 1,176,020,000