Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Current assets: [Abstract]
Cash and cash equivalents	$ 6,230	$ 4,211	$ 5,227
Accounts receivable	25,506	68,228	29,800
Accrued unbilled revenue	15,537	57,987	15,752
Allowance for uncollectible accounts	(289)	(870)	(308)
Regulatory Assets, Current	55,280	69,178	82,712
Derivative instruments	4,857	2,719	2,956
Inventories	67,470	70,868	80,974
Deferred Gas Cost	16,257	17,094	17,822
Income taxes receivable	2,257	7,900	0
Deferred tax assets	0	0	15,663
Other current assets	17,480	33,460	25,972
Total current assets	210,585	330,775	276,570
Non-current assets: [Abstract]
Property, plant, and equipment	3,177,196	3,089,380	3,072,998
Less: Accumulated depreciation	943,334	906,717	905,137
Total property, plant, and equipment, net	2,233,862	2,182,663	2,167,861
Inventory, Gas in Storage Underground, Noncurrent	103,976	114,552	117,784
Regulatory assets	341,188	370,711	333,953
Derivative instruments	1,151	27	299
Other investments	67,853	68,066	68,503
Restricted cash	0	0	4,500
Other non-current assets	1,269	2,616	1,248
Total non-current assets	2,749,299	2,738,635	2,694,148
Total assets	2,959,884	3,069,410	2,970,718
Current liabilities: [Abstract]
Short-term Debt	194,900	270,035	225,200
Current maturities of long-term debt	64,994	24,973	0
Accounts payable	55,933	73,219	54,425
Taxes accrued	11,954	10,420	11,854
Interest accrued	9,671	5,873	9,800
Regulatory liabilities	27,921	29,927	34,127
Derivative instruments	5,334	22,092	21,949
Other current liabilities	31,997	41,148	27,924
Total current liabilities	402,704	477,687	385,279
Long-term debt	530,219	569,445	614,053
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	544,575	530,021	527,336
Regulatory liabilities	342,143	339,287	334,490
Pension and other postretirement benefit liabilities	216,909	223,105	228,861
Derivative instruments	1,682	3,447	3,540
Other non-current liabilities	142,450	145,446	117,950
Total deferred credits and other non-current liabilities	1,247,759	1,241,306	1,212,177
Commitments and contingencies (see Note 13)	0	0	0
Equity: [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,558, 27,367, and 27,427 at September 30, 2016 and 2015, and December 31, 2015, respectively	389,834	383,144	380,208
Retained earnings	396,938	404,990	388,082
Accumulated other comprehensive loss	(7,570)	(7,162)	(9,081)
Total equity	779,202	780,972	759,209
Total liabilities and equity	$ 2,959,884	$ 3,069,410	$ 2,970,718