Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Current assets: [Abstract]
Cash and cash equivalents	$ 9,471	$ 8,923
Accounts receivable	81,889	61,229
Accrued unbilled revenue	61,527	56,955
Allowance for uncollectible accounts	(1,656)	(2,518)
Regulatory assets	22,635	52,448
Derivative instruments	5,311	1,950
Inventories	60,669	67,602
Gas reserves	20,646	14,966
Income taxes receivable	3,534	2,552
Deferred tax assets	45,241	0
Other current assets	21,181	19,592
Total current assets	330,448	283,699
Non-current assets: [Abstract]
Property, plant, and equipment	2,918,739	2,786,008
Less: Accumulated depreciation	855,865	812,396
Total property, plant, and equipment, net	2,062,874	1,973,612
Gas reserves	121,998	84,693
Regulatory assets	369,603	382,255
Derivative instruments	1,880	3,639
Other investments	67,851	67,667
Restricted cash	4,000	4,000
Other non-current assets	12,257	13,555
Total non-current assets	2,640,463	2,529,421
Total assets	2,970,911	2,813,120
Current liabilities: [Abstract]
Short-term debt	188,200	190,250
Current maturities of long-term debt	60,000	0
Accounts payable	96,126	85,613
Taxes accrued	10,856	9,588
Interest accrued	7,103	5,953
Regulatory liabilities	28,335	20,792
Derivative instruments	1,891	10,796
Other current liabilities	40,280	45,444
Total current liabilities	432,791	368,436
Long-term debt	681,700	691,700
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	532,036	444,377
Regulatory liabilities	303,485	288,113
Pension and other postretirement benefit liabilities	149,354	215,792
Derivative instruments	615	578
Other non-current liabilities	119,058	74,497
Total deferred credits and other non-current liabilities	1,104,548	1,023,357
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 27,075 and 26,917 at December 31, 2013 and 2012, respectively	364,549	356,571
Retained earnings	393,681	382,347
Accumulated other comprehensive loss	(6,358)	(9,291)
Total equity	751,872	729,627
Total liabilities and equity	$ 2,970,911	$ 2,813,120