Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Current assets [Abstract]
Cash and cash equivalents	$ 17,240	$ 9,471	$ 12,214
Accounts receivable	38,621	81,889	39,061
Accrued unbilled revenue	14,592	61,527	14,692
Allowance for uncollectible accounts	(1,404)	(1,656)	(1,189)
Regulatory assets	38,265	22,635	25,952
Derivative instruments	11,191	5,311	623
Inventories	60,808	60,669	62,412
Gas reserves	20,373	20,646	15,324
Income taxes receivable	0	3,534	1,297
Deferred tax assets	4,915	45,241	0
Other current assets	14,518	21,181	8,781
Total current assets	219,119	330,448	179,167
Non-current assets [Abstract]
Property, plant, and equipment	2,965,226	2,918,739	2,833,083
Less: Accumulated depreciation	879,296	855,865	833,851
Total property, plant, and equipment, net	2,085,930	2,062,874	1,999,232
Gas reserves	130,280	121,998	113,762
Regulatory assets	267,248	369,603	393,652
Derivative instruments	1,202	1,880	1,054
Other investments	67,689	67,851	67,410
Restricted cash	3,000	4,000	4,000
Other non-current assets	12,646	12,257	14,312
Total non-current assets	2,567,995	2,640,463	2,593,422
Total assets	2,787,114	2,970,911	2,772,589
Current liabilities [Abstract]
Short-term debt	74,200	188,200	136,000
Current maturities of long-term debt	100,000	60,000	0
Accounts payable	68,973	96,126	63,466
Taxes accrued	15,769	10,856	6,798
Interest accrued	7,053	7,103	6,404
Regulatory liabilities	26,742	28,335	16,644
Derivative instruments	1,490	1,891	9,392
Other current liabilities	34,507	40,280	34,446
Total current liabilities	328,734	432,791	273,150
Long-term debt	621,700	681,700	691,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	489,892	532,036	469,964
Regulatory liabilities	309,327	303,485	294,202
Pension and other postretirement benefit liabilities	145,861	149,354	214,125
Derivative instruments	191	615	1,754
Other non-current liabilities	120,423	119,058	79,145
Total deferred credits and other non-current liabilities	1,065,694	1,104,548	1,059,190
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,147, 26,972, and 27,075 at June 30, 2014 and 2013 and December 31, 2013, respectively	369,315	364,549	359,772
Retained earnings	407,698	393,681	397,603
Accumulated other comprehensive loss	(6,027)	(6,358)	(8,826)
Total equity	770,986	751,872	748,549
Total liabilities and equity	$ 2,787,114	$ 2,970,911	$ 2,772,589