Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Current assets [Abstract]
Cash and cash equivalents	$ 8,275	$ 9,471	$ 16,105
Accounts receivable	30,468	81,889	29,821
Accrued unbilled revenue	12,442	61,527	16,493
Allowance for uncollectible accounts	(840)	(1,656)	(802)
Regulatory assets	52,250	22,635	26,293
Derivative instruments	5,587	5,311	1,452
Inventories	86,600	60,669	75,419
Gas reserves	21,455	20,646	18,083
Income taxes receivable	7,639	3,534	909
Deferred tax assets	5,100	45,241	0
Other current assets	19,158	21,181	11,936
Total current assets	248,134	330,448	195,709
Non-current assets [Abstract]
Property, plant, and equipment	2,990,662	2,918,739	2,865,860
Less: Accumulated depreciation	883,568	855,865	846,346
Total property, plant, and equipment, net	2,107,094	2,062,874	2,019,514
Gas reserves	131,745	121,998	115,218
Regulatory assets	263,321	369,603	387,676
Derivative instruments	602	1,880	1,682
Other investments	67,980	67,851	67,548
Restricted cash	3,000	4,000	4,000
Other non-current assets	11,648	12,257	14,566
Total non-current assets	2,585,390	2,640,463	2,610,204
Total assets	2,833,524	2,970,911	2,805,913
Current liabilities [Abstract]
Short-term debt	190,000	188,200	141,300
Current maturities of long-term debt	40,000	60,000	60,000
Accounts payable	71,018	96,126	67,652
Taxes accrued	11,876	10,856	11,302
Interest accrued	10,427	7,103	11,143
Regulatory liabilities	23,352	28,335	16,506
Derivative instruments	5,520	1,891	8,275
Other current liabilities	33,481	40,280	26,289
Total current liabilities	385,674	432,791	342,467
Long-term debt	621,700	681,700	681,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	499,809	532,036	463,566
Regulatory liabilities	312,500	303,485	298,220
Pension and other postretirement benefit liabilities	142,502	149,354	210,943
Derivative instruments	551	615	1,404
Other non-current liabilities	118,531	119,058	77,322
Total deferred credits and other non-current liabilities	1,073,893	1,104,548	1,051,455
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,203, 27,001, and 27,075 at September 30, 2014 and 2013 and December 31, 2013, respectively	371,657	364,549	361,789
Retained earnings	386,461	393,681	377,096
Accumulated other comprehensive loss	(5,861)	(6,358)	(8,594)
Total equity	752,257	751,872	730,291
Total liabilities and equity	$ 2,833,524	$ 2,970,911	$ 2,805,913