Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Current assets: [Abstract]
Cash and cash equivalents	$ 4,321	$ 4,211	$ 5,218
Accounts receivable	69,066	68,228	68,531
Accrued unbilled revenue	36,393	57,987	30,076
Allowance for uncollectible accounts	(1,376)	(870)	(1,363)
Regulatory Assets, Current	61,524	69,178	67,702
Derivative instruments	1,960	2,719	658
Inventories	60,581	70,868	69,289
Deferred Gas Cost	16,420	17,094	19,112
Income taxes receivable	0	7,900	2,000
Deferred tax assets	0	0	13,491
Other current assets	23,311	33,460	15,921
Total current assets	272,200	330,775	290,635
Non-current assets: [Abstract]
Property, plant, and equipment	3,115,854	3,089,380	3,017,754
Less: Accumulated depreciation	919,187	906,717	883,254
Total property, plant, and equipment, net	2,196,667	2,182,663	2,134,500
Inventory, Gas in Storage Underground, Noncurrent	111,145	114,552	125,187
Regulatory assets	351,390	370,711	348,421
Derivative instruments	452	27	117
Other investments	67,490	68,066	68,614
Restricted cash	0	0	3,000
Other non-current assets	2,689	2,616	3,638
Total non-current assets	2,729,833	2,738,635	2,683,477
Total assets	3,002,033	3,069,410	2,974,112
Current liabilities: [Abstract]
Short-term Debt	164,900	270,035	156,200
Current maturities of long-term debt	24,980	24,973	39,994
Accounts payable	57,407	73,219	62,904
Taxes accrued	10,256	10,420	17,755
Interest accrued	9,671	5,873	10,427
Regulatory liabilities	35,596	29,927	24,263
Derivative instruments	17,313	22,092	23,242
Other current liabilities	42,100	41,148	35,950
Total current liabilities	362,223	477,687	370,735
Long-term debt	569,745	569,445	613,417
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	550,731	530,021	523,929
Regulatory liabilities	346,761	339,287	326,424
Pension and other postretirement benefit liabilities	221,291	223,105	235,516
Derivative instruments	1,237	3,447	1,117
Other non-current liabilities	143,090	145,446	118,059
Total deferred credits and other non-current liabilities	1,263,110	1,241,306	1,205,045
Commitments and contingencies (see Note 13)	0	0	0
Equity: [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,493, 27,332, and 27,427 at March 31, 2016 and 2015, and December 31, 2015, respectively	385,232	383,144	376,656
Retained earnings	428,691	404,990	418,003
Accumulated other comprehensive loss	(6,968)	(7,162)	(9,744)
Total equity	806,955	780,972	784,915
Total liabilities and equity	$ 3,002,033	$ 3,069,410	$ 2,974,112