Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Current assets [Abstract]
Cash and cash equivalents	$ 16,105	$ 8,923	$ 5,718
Accounts receivable	29,821	61,229	23,382
Allowance for uncollectible accounts	(802)	(2,518)	(1,985)
Accrued unbilled revenue	16,493	56,955	11,184
Regulatory assets	26,293	52,448	53,891
Derivative instruments	1,452	1,950	6,771
Inventories	75,419	67,602	73,188
Gas reserves	18,083	14,966	13,140
Income taxes receivable	909	2,552	1,787
Other current assets	11,936	19,592	10,825
Total current assets	195,709	283,699	197,901
Non-current assets [Abstract]
Property, plant, and equipment	2,865,860	2,786,008	2,755,729
Less: Accumulated depreciation	846,346	812,396	798,510
Total property, plant, and equipment, net	2,019,514	1,973,612	1,957,219
Gas reserves	115,218	84,693	75,925
Regulatory assets	387,676	382,255	362,472
Derivative instruments	1,682	3,639	5,608
Other investments	67,548	67,667	67,333
Restricted cash	4,000	4,000	4,000
Other non-current assets	14,566	13,555	14,690
Total non-current assets	2,610,204	2,529,421	2,487,247
Total assets	2,805,913	2,813,120	2,685,148
Current liabilities [Abstract]
Short-term debt	141,300	190,250	175,800
Current maturities of long-term debt	60,000	0	0
Accounts payable	67,652	85,613	61,327
Taxes accrued	11,302	9,588	10,269
Interest accrued	11,143	5,953	10,593
Regulatory liabilities	16,506	20,792	24,810
Derivative instruments	8,275	10,796	17,156
Other current liabilities	26,289	45,444	45,425
Total current liabilities	342,467	368,436	345,380
Long-term debt	681,700	691,700	641,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	463,566	444,377	428,821
Regulatory liabilities	298,220	288,113	288,097
Pension and other postretirement benefit liabilities	210,943	215,792	182,069
Derivative instruments	1,404	578	615
Other non-current liabilities	77,322	74,497	84,063
Total deferred credits and other non-current liabilities	1,051,455	1,023,357	983,665
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,001, 26,866, and 26,917 at September 30, 2013 and 2012 and December 31, 2012, respectively	361,789	356,571	355,276
Retained earnings	377,096	382,347	366,428
Accumulated other comprehensive loss	(8,594)	(9,291)	(7,301)
Total equity	730,291	729,627	714,403
Total liabilities and equity	$ 2,805,913	$ 2,813,120	$ 2,685,148