Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current assets: [Abstract]
Cash and cash equivalents	$ 9,534	$ 9,471
Accounts receivable	69,818	81,889
Accrued unbilled revenue	57,963	61,527
Allowance for uncollectible accounts	(969)	(1,656)
Regulatory assets	68,562	22,635
Derivative instruments	243	5,311
Inventories	77,832	60,669
Gas reserves	20,020	20,646
Income taxes receivable	1,000	3,534
Deferred tax assets	23,785	45,241
Other current assets	34,772	21,181
Total current assets	362,560	330,448
Non-current assets: [Abstract]
Property, plant, and equipment	2,992,560	2,918,739
Less: Accumulated depreciation	870,967	855,865
Total property, plant, and equipment, net	2,121,593	2,062,874
Gas reserves	129,280	121,998
Regulatory assets	368,908	369,603
Derivative instruments	0	1,880
Other investments	68,238	67,851
Restricted cash	3,000	4,000
Other non-current assets	11,366	12,257
Total non-current assets	2,702,385	2,640,463
Total assets	3,064,945	2,970,911
Current liabilities: [Abstract]
Short-term debt	234,700	188,200
Current maturities of long-term debt	40,000	60,000
Accounts payable	91,366	96,126
Taxes accrued	10,031	10,856
Interest accrued	6,079	7,103
Regulatory liabilities	19,105	28,335
Derivative instruments	29,894	1,891
Other current liabilities	38,235	40,280
Total current liabilities	469,410	432,791
Long-term debt	621,700	681,700
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	530,965	532,036
Regulatory liabilities	317,205	303,485
Pension and other postretirement benefit liabilities	236,735	149,354
Derivative instruments	3,515	615
Other non-current liabilities	118,094	119,058
Total deferred credits and other non-current liabilities	1,206,514	1,104,548
Commitments and contingencies (see Note 14 and Note 15)	0	0
Equity: [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,284 and 27,075 at December 31, 2014 and 2013, respectively	375,117	364,549
Retained earnings	402,280	393,681
Accumulated other comprehensive loss	(10,076)	(6,358)
Total equity	767,321	751,872
Total liabilities and equity	$ 3,064,945	$ 2,970,911