Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Current assets [Abstract]
Cash and cash equivalents	$ 17,929	$ 9,471	$ 8,337
Accounts receivable	87,264	81,889	84,346
Accrued unbilled revenue	33,515	61,527	29,633
Allowance for uncollectible accounts	(2,235)	(1,656)	(2,116)
Regulatory assets	27,834	22,635	39,001
Derivative instruments	15,846	5,311	8,200
Inventories	33,469	60,669	52,004
Gas reserves	21,990	20,646	14,286
Income taxes receivable	0	3,534	2,033
Deferred tax assets	4,915	45,241	0
Other current assets	13,595	21,181	12,441
Total current assets	254,122	330,448	248,165
Non-current assets [Abstract]
Property, plant, and equipment	2,939,760	2,918,739	2,808,673
Less: Accumulated depreciation	868,257	855,865	824,561
Total property, plant, and equipment, net	2,071,503	2,062,874	1,984,112
Gas reserves	134,894	121,998	100,169
Regulatory assets	285,046	369,603	384,453
Derivative instruments	1,078	1,880	2,836
Other investments	67,288	67,851	68,029
Restricted cash	4,000	4,000	4,000
Other non-current assets	12,453	12,257	14,735
Total non-current assets	2,576,262	2,640,463	2,558,334
Total assets	2,830,384	2,970,911	2,806,499
Current liabilities [Abstract]
Short-term debt	32,600	188,200	130,750
Current maturities of long-term debt	80,000	60,000	0
Accounts payable	89,201	96,126	77,007
Taxes accrued	34,146	10,856	10,262
Interest accrued	11,144	7,103	10,952
Regulatory liabilities	37,686	28,335	28,239
Derivative instruments	1,191	1,891	3,450
Other current liabilities	38,069	40,280	41,445
Total current liabilities	324,037	432,791	302,105
Long-term debt	661,700	681,700	691,700
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	489,108	532,036	467,360
Regulatory liabilities	308,858	303,485	293,135
Pension and other postretirement benefit liabilities	147,733	149,354	215,808
Derivative instruments	96	615	642
Other non-current liabilities	119,376	119,058	79,112
Total deferred credits and other non-current liabilities	1,065,171	1,104,548	1,056,057
Commitments and contingencies (see Note 13)	0	0	0
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 27,132, 26,948, and 27,075 at March 31, 2014 and 2013 and December 31, 2013, respectively	366,560	364,549	357,957
Retained earnings	419,109	393,681	407,738
Accumulated other comprehensive loss	(6,193)	(6,358)	(9,058)
Total equity	779,476	751,872	756,637
Total liabilities and equity	$ 2,830,384	$ 2,970,911	$ 2,806,499