Summary of Significant Accounting Policies (details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	$ 59,766	$ 52,714	$ 56,804
Regulatory Assets	326,081	348,897	329,197
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	25,784	17,828	29,524
Regulatory Liabilities	265,703	258,031	251,585
Unrealized Loss On Derivatives [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	25,986	38,437	34,463
Regulatory Assets	9,202	17,022	16,917
Pension and Other Postretirement Benefit Liabilities [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	10,988	10,988	7,502
Regulatory Assets	112,743	118,248	106,089
Other Regulatory [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	22,792	3,289	14,839
Regulatory Assets	13,610	26,975	21,352
Other Regulatory [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	3,813	0	4,613
Regulatory Liabilities	2,045	2,165	2,075
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	120,285	114,311	109,324
Gas Costs Payable [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	17,538	15,583	23,416
Regulatory Liabilities	3,023	2,297	2,218
Unrealized Gain On Derivatives [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	4,433	2,245	1,495
Regulatory Liabilities	1,042	628	453
Accrued Asset Removal Costs [Member]
Table Of Regulatory Liabilities [Abstract]
Regulatory Liabilities	$ 259,593	$ 252,941	$ 246,839