Summary of Significant Accounting Policies (details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	$ 76,734	$ 52,714	$ 83,545
Regulatory Assets	328,757	348,897	339,786
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	28,593	17,828	31,502
Regulatory Liabilities	266,907	258,031	252,425
Unrealized Loss On Derivatives [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	46,651	38,437	59,898
Regulatory Assets	7,429	17,022	27,211
Pension and Other Postretirement Benefit Liabilities [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	10,988	10,988	7,502
Regulatory Assets	110,007	118,248	104,327
Other Regulatory [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	19,095	3,289	16,145
Regulatory Assets	18,626	26,975	20,802
Other Regulatory [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	7,670	0	9,151
Regulatory Liabilities	2,307	2,165	2,087
Income Tax Asset [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets	70,241	72,341	75,515
Environmental Costs [Member]
Table Of Regulatory Assets [Abstract]
Regulatory Assets	122,454	114,311	111,931
Gas Costs Payable [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	16,991	15,583	20,487
Regulatory Liabilities	1,250	2,297	900
Unrealized Gain On Derivatives [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	3,932	2,245	1,864
Regulatory Liabilities	227	628	518
Accrued Asset Removal Costs [Member]
Table Of Regulatory Liabilities [Abstract]
Regulatory Liabilities	$ 263,123	$ 252,941	$ 248,920