Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current assets [Abstract]
Cash and cash equivalents	$ 8,923	$ 5,833
Accounts receivable	61,229	77,449
Accrued unbilled revenue	56,955	61,925
Allowance for uncollectible accounts	(2,518)	(2,895)
Regulatory assets	52,448	94,673
Derivative instruments	1,950	2,853
Inventories	67,602	74,363
Gas reserves	14,966	4,463
Income taxes receivable	2,552	7,045
Other current assets	19,592	22,980
Total current assets	283,699	348,689
Non-current assets [Abstract]
Property, plant, and equipment	2,786,008	2,661,102
Less: Accumulated depreciation	812,396	767,226
Total property, plant, and equipment, net	1,973,612	1,893,876
Gas reserves	84,693	47,451
Regulatory assets	387,888	371,392
Derivative instruments	3,639	0
Other investments	67,667	68,263
Restricted cash	4,000	4,000
Other non-current assets	13,555	12,903
Total non-current assets	2,535,054	2,397,885
Total assets	2,818,753	2,746,574
Current liabilities [Abstract]
Short-term debt	190,250	141,600
Current maturities of long-term debt	0	40,000
Accounts payable	85,613	86,300
Taxes accrued	9,588	10,747
Interest accrued	5,953	5,857
Regulatory liabilities	20,792	31,046
Derivative instruments	10,796	57,317
Other current liabilities	45,444	41,597
Total current liabilities	368,436	414,464
Long-term debt	691,700	641,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	446,604	413,209
Regulatory liabilities	288,113	278,382
Pension and other postretirement benefit liabilities	215,792	201,530
Derivative instruments	578	6,536
Other non-current liabilities	74,497	76,265
Total deferred credits and other non-current liabilities	1,025,584	975,922
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 26,917 and 26,756 at December 31, 2012 and 2011, respectively	356,571	348,383
Retained earnings	385,753	373,905
Accumulated other comprehensive loss	(9,291)	(7,800)
Total equity	733,033	714,488
Total liabilities and equity	$ 2,818,753	$ 2,746,574