Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Current assets [Abstract]
Cash and cash equivalents	$ 5,718	$ 5,833	$ 25,862
Accounts receivable	23,382	77,449	25,628
Accrued unbilled revenue	11,184	61,925	14,287
Allowance for uncollectible accounts	(1,985)	(2,895)	(1,733)
Regulatory assets	53,891	94,673	76,734
Derivative instruments	6,771	2,853	3,932
Inventories	73,188	74,363	83,581
Gas reserves	13,140	4,463	2,366
Income taxes receivable	1,787	7,045	5,019
Other current assets	10,825	22,980	14,871
Total current assets	197,901	348,689	250,547
Non-current assets [Abstract]
Property, plant and equipment	2,755,729	2,661,102	2,632,498
Accumulated depreciation	798,510	767,226	756,592
Total property, plant and equipment - net	1,957,219	1,893,876	1,875,906
Gas reserves	75,925	47,451	28,125
Regulatory assets	367,692	371,392	328,757
Derivative instruments	5,608	0	227
Other investments	67,333	68,263	69,022
Restricted cash	4,000	4,000	0
Other non-current assets	14,690	12,903	15,256
Total non-current assets	2,492,467	2,397,885	2,317,293
Total assets	2,690,368	2,746,574	2,567,840
Capitalization [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 26,866, 26,703, and 26,756 at September 30, 2012 and 2011 and December 31, 2011, respectively	355,276	348,383	346,197
Retained earnings	369,584	373,905	356,574
Accumulated other comprehensive loss	(7,301)	(7,800)	(6,166)
Total common stock equity	717,559	714,488	696,605
Long-term debt	641,700	641,700	601,700
Total capitalization	1,359,259	1,356,188	1,298,305
Current liabilities [Abstract]
Short-term debt	175,800	141,600	181,200
Current maturities of long-term debt	0	40,000	40,000
Accounts payable	61,327	86,300	50,117
Taxes accrued	10,269	10,747	11,117
Interest accrued	10,593	5,857	11,321
Regulatory liabilities	24,810	31,046	28,593
Derivative instruments	17,156	57,317	46,651
Other current liabilities	45,425	41,597	33,609
Total current liabilities	345,380	414,464	402,608
Deferred credits and other non-current liabilities:
Deferred tax liabilities	430,885	413,209	394,217
Regulatory liabilities	288,097	278,382	266,907
Pension and other postretirement benefit liabilities	182,069	201,530	129,669
Derivative instruments	615	6,536	7,429
Other non-current liabilities	84,063	76,265	68,705
Total deferred credits and other non-current liabilities	985,729	975,922	866,927
Total capitalization and liabilities	$ 2,690,368	$ 2,746,574	$ 2,567,840