Summary of Significant Accounting Policies (details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	$ 65,297	$ 94,673	$ 59,766
Regulatory assets	366,981	371,392	326,081
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	20,748	31,046	25,784
Regulatory liabilities	280,295	278,382	265,703
Gas Costs Payable [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	12,980	17,994	17,538
Regulatory liabilities	1,504	8,420	3,023
Unrealized Gain On Derivatives [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	2,142	2,853	4,433
Regulatory liabilities	1,170	0	1,042
Other Regulatory [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	5,626	10,199	3,813
Regulatory liabilities	2,865	2,607	2,045
Accrued Asset Removal Costs [Member]
Table Of Regulatory Liabilities [Abstract]
Regulatory liabilities	274,756	267,355	259,593
Unrealized Loss On Derivatives [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	29,407	57,317	25,986
Regulatory assets	2,130	6,536	9,202
Pension and Other Postretirement Benefit Liabilities [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	15,491	15,491	10,988
Regulatory assets	162,767	170,512	112,743
Other Regulatory [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	20,399	21,865	22,792
Regulatory assets	9,961	17,402	10,951
Pension Balancing [Member]
Table Of Regulatory Assets [Abstract]
Regulatory assets	10,766	6,008	2,659
Income Tax Asset [Member]
Table Of Regulatory Assets [Abstract]
Regulatory assets	63,452	65,264	70,241
Environmental Costs [Member]
Table Of Regulatory Assets [Abstract]
Regulatory assets	$ 117,905	$ 105,670	$ 120,285