Consolidated Balance Sheets (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Current assets [Abstract]
Cash and cash equivalents	$ 8,337,000	$ 8,923,000	$ 4,031,000
Accounts receivable	84,346,000	61,229,000	90,817,000
Accrued unbilled revenue	29,633,000	56,955,000	44,444,000
Allowance for uncollectible accounts	(2,116,000)	(2,518,000)	(3,694,000)
Regulatory assets	39,001,000	52,448,000	90,490,000
Derivative instruments	8,200,000	1,950,000	1,824,000
Inventories	52,004,000	67,602,000	61,436,000
Gas reserves	14,286,000	14,966,000	6,732,000
Income taxes receivable	2,033,000	2,552,000	1,735,000
Other current assets	12,441,000	19,592,000	13,075,000
Total current assets	248,165,000	283,699,000	310,890,000
Non-current assets [Abstract]
Property, plant, and equipment	2,808,673,000	2,786,008,000	2,680,537,000
Less: Accumulated depreciation	824,561,000	812,396,000	779,683,000
Total property, plant, and equipment, net	1,984,112,000	1,973,612,000	1,900,854,000
Gas reserves	100,169,000	84,693,000	61,106,000
Regulatory assets	384,453,000	382,255,000	364,132,000
Derivative instruments	2,836,000	3,639,000	52,000
Other investments	68,029,000	67,667,000	67,648,000
Restricted cash	4,000,000	4,000,000	4,000,000
Other non-current assets	14,735,000	13,555,000	14,191,000
Total non-current assets	2,558,334,000	2,529,421,000	2,411,983,000
Total assets	2,806,499,000	2,813,120,000	2,722,873,000
Current liabilities [Abstract]
Short-term debt	130,750,000	190,250,000	113,700,000
Accounts payable	77,007,000	85,613,000	60,165,000
Taxes accrued	10,262,000	9,588,000	10,509,000
Interest accrued	10,952,000	5,953,000	10,648,000
Regulatory liabilities	28,239,000	20,792,000	50,341,000
Derivative instruments	3,450,000	10,796,000	53,697,000
Other current liabilities	41,445,000	45,444,000	41,503,000
Total current liabilities	302,105,000	368,436,000	340,563,000
Long-term debt	691,700,000	691,700,000	641,700,000
Deferred credits and other non-current liabilities [Abstract]
Deferred tax liabilities	467,360,000	444,377,000	436,750,000
Regulatory liabilities	293,135,000	288,113,000	288,131,000
Pension and other postretirement benefit liabilities	215,808,000	215,792,000	189,003,000
Derivative instruments	642,000	578,000	3,947,000
Other non-current liabilities	79,112,000	74,497,000	79,461,000
Total deferred credits and other non-current liabilities	1,056,057,000	1,023,357,000	997,292,000
Equity [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 26,948, 26,798, and 26,917 at March 31, 2013 and 2012 and December 31, 2012, respectively	357,957,000	356,571,000	351,005,000
Retained earnings	407,738,000	382,347,000	399,946,000
Accumulated other comprehensive loss	(9,058,000)	(9,291,000)	(7,633,000)
Total equity	756,637,000	729,627,000	743,318,000
Commitments and contingencies (see Note 13)	0	0	0
Total liabilities and equity	$ 2,806,499,000	$ 2,813,120,000	$ 2,722,873,000