Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Current assets [Abstract]
Cash and cash equivalents	$ 12,214	$ 8,923	$ 4,002
Accounts receivable	39,061	61,229	13,459
Accrued unbilled revenue	14,692	56,955	12,921
Allowance for uncollectible accounts	(1,189)	(2,518)	(2,653)
Regulatory assets	25,952	52,448	65,297
Derivative instruments	623	1,950	2,142
Inventories	62,412	67,602	68,868
Gas reserves	15,324	14,966	11,021
Income taxes receivable	1,297	2,552	3,119
Other current assets	8,781	19,592	8,606
Total current assets	179,167	283,699	186,782
Non-current assets [Abstract]
Property, plant, and equipment	2,833,083	2,786,008	2,720,037
Less: Accumulated depreciation	833,851	812,396	791,021
Total property, plant, and equipment, net	1,999,232	1,973,612	1,929,016
Gas reserves	113,762	84,693	65,026
Regulatory assets	393,652	382,255	362,290
Derivative instruments	1,054	3,639	1,170
Other investments	67,410	67,667	68,230
Restricted cash	4,000	4,000	4,000
Other non-current assets	14,312	13,555	13,936
Total non-current assets	2,593,422	2,529,421	2,443,668
Total assets	2,772,589	2,813,120	2,630,450
Current liabilities [Abstract]
Short-term debt	136,000	190,250	113,200
Accounts payable	63,466	85,613	48,361
Taxes accrued	6,798	9,588	5,205
Interest accrued	6,404	5,953	5,607
Regulatory liabilities	16,644	20,792	20,748
Derivative instruments	9,392	10,796	29,407
Other current liabilities	34,446	45,444	42,336
Total current liabilities	273,150	368,436	264,864
Long-term debt	691,700	691,700	641,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	469,964	444,377	438,217
Regulatory liabilities	294,202	288,113	280,295
Pension and other postretirement benefit liabilities	214,125	215,792	185,844
Derivative instruments	1,754	578	2,130
Other non-current liabilities	79,145	74,497	82,665
Total deferred credits and other non-current liabilities	1,059,190	1,023,357	989,151
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 26,972, 26,827, and 26,917 at June 30, 2013 and 2012 and December 31, 2012, respectively	359,772	356,571	352,955
Retained earnings	397,603	382,347	389,247
Accumulated other comprehensive loss	(8,826)	(9,291)	(7,467)
Total equity	748,549	729,627	734,735
Total liabilities and equity	$ 2,772,589	$ 2,813,120	$ 2,630,450