Summary of Significant Accounting Policies (details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	$ 90,490	$ 94,673	$ 48,195
Regulatory Assets	368,521	371,392	345,452
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	50,341	31,046	29,016
Regulatory Liabilities	288,131	278,382	263,876
Gas Costs Payable [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	35,584	17,994	14,144
Regulatory Liabilities	14,462	8,420	3,932
Unrealized Gain On Derivatives [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	1,824	2,853	4,861
Regulatory Liabilities	52	0	1,560
Other Regulatory [Member]
Table Of Regulatory Liabilities [Abstract]
Current Regulatory Liabilities	12,933	10,199	10,011
Regulatory Liabilities	2,780	2,607	2,181
Accrued Asset Removal Costs [Member]
Table Of Regulatory Liabilities [Abstract]
Regulatory Liabilities	270,837	267,355	256,203
Unrealized Loss On Derivatives [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	53,697	57,317	25,655
Regulatory Assets	3,947	6,536	13,914
Pension and Other Postretirement Benefit Liabilities [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	15,491	15,491	10,988
Regulatory Assets	166,639	170,512	115,490
Other Regulatory [Member]
Table Of Regulatory Assets [Abstract]
Current Regulatory Assets	21,302	21,865	11,552
Regulatory Assets	22,186	23,410	28,263
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	$ 112,297	$ 105,670	$ 117,544