Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Current assets [Abstract]
Cash and cash equivalents	$ 5,227	$ 9,534	$ 8,275
Accounts receivable	29,800	69,818	30,468
Accrued unbilled revenue	15,752	57,963	12,442
Allowance for uncollectible accounts	(308)	(969)	(840)
Regulatory assets	82,712	68,562	52,250
Derivative instruments	2,956	243	5,587
Inventories	80,974	77,832	86,600
Gas reserves	17,822	20,020	21,455
Income taxes receivable	0	1,000	7,639
Deferred tax assets	15,663	23,785	5,100
Other current assets	27,313	34,772	19,158
Total current assets	277,911	362,560	248,134
Non-current assets [Abstract]
Property, plant, and equipment	3,072,998	2,992,560	2,990,662
Less: Accumulated depreciation	905,137	870,967	883,568
Total property, plant, and equipment, net	2,167,861	2,121,593	2,107,094
Gas reserves	117,784	129,280	131,745
Regulatory assets, Noncurrent	333,953	368,908	263,321
Derivative instruments	299	0	602
Other investments	68,503	68,238	67,980
Restricted cash	4,500	3,000	3,000
Other non-current assets	7,554	11,366	11,648
Total non-current assets	2,700,454	2,702,385	2,585,390
Total assets	2,978,365	3,064,945	2,833,524
Current liabilities [Abstract]
Short-term debt	225,200	234,700	190,000
Current maturities of long-term debt	0	40,000	40,000
Accounts payable	54,425	91,366	71,018
Taxes accrued	11,854	10,031	11,876
Interest accrued	9,800	6,079	10,427
Regulatory liabilities	34,127	19,105	23,352
Derivative instruments	21,949	29,894	5,520
Other current liabilities	27,924	38,235	33,481
Total current liabilities	385,279	469,410	385,674
Long-term debt	621,700	621,700	621,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	527,336	530,965	499,809
Regulatory liabilities	334,490	317,205	312,500
Pension and other postretirement benefit liabilities	228,861	236,735	142,502
Derivative instruments	3,540	3,515	551
Other non-current liabilities	117,950	118,094	118,531
Total deferred credits and other non-current liabilities	1,212,177	1,206,514	1,073,893
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,367, 27,203, and 27,284 at September 30, 2015 and 2014 and December 31, 2014, respectively	380,208	375,117	371,657
Retained earnings	388,082	402,280	386,461
Accumulated other comprehensive loss	(9,081)	(10,076)	(5,861)
Total equity	759,209	767,321	752,257
Total liabilities and equity	$ 2,978,365	$ 3,064,945	$ 2,833,524