Consolidated Balance Sheets - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Statement of Financial Position [Abstract]
Document Fiscal Year Focus	2017
Current assets: [Abstract]
Cash and cash equivalents	$ 40,639,000	$ 3,521,000	$ 4,321,000
Accounts receivable	70,429,000	66,700,000	69,066,000
Accrued unbilled revenue	38,017,000	64,946,000	36,393,000
Allowance for uncollectible accounts	(1,668,000)	(1,290,000)	(1,376,000)
Regulatory Assets, Current	34,874,000	42,362,000	61,524,000
Derivative instruments	2,908,000	17,031,000	1,960,000
Inventories	48,484,000	54,129,000	60,581,000
Deferred Gas Cost	15,378,000	15,926,000	16,420,000
Other current assets	16,832,000	24,728,000	23,311,000
Total current assets	265,893,000	288,053,000	272,200,000
Non-current assets: [Abstract]
Property, plant, and equipment	3,247,177,000	3,208,816,000	3,115,854,000
Less: Accumulated depreciation	960,336,000	947,916,000	919,187,000
Total property, plant, and equipment, net	2,286,841,000	2,260,900,000	2,196,667,000
Inventory, Gas in Storage Underground, Noncurrent	96,630,000	100,184,000	111,145,000
Regulatory assets	349,057,000	357,530,000	351,390,000
Derivative instruments	46,000	3,265,000	452,000
Other investments	68,729,000	68,376,000	67,490,000
Other non-current assets	3,460,000	1,493,000	2,689,000
Total non-current assets	2,804,763,000	2,791,748,000	2,729,833,000
Total assets	3,070,656,000	3,079,801,000	3,002,033,000
Current liabilities: [Abstract]
Short-term Debt	0	53,300,000	164,900,000
Current maturities of long-term debt	61,994,000	39,989,000	24,980,000
Accounts payable	73,245,000	85,664,000	57,407,000
Taxes accrued	16,653,000	12,149,000	10,256,000
Interest accrued	10,581,000	5,966,000	9,671,000
Regulatory liabilities	33,211,000	40,290,000	35,596,000
Derivative instruments	1,638,000	1,315,000	17,313,000
Other current liabilities	37,697,000	35,844,000	42,100,000
Total current liabilities	235,019,000	274,517,000	362,223,000
Long-term debt	657,716,000	679,334,000	569,745,000
Deferred credits and other non-current liabilities: [Abstract]
Deferred tax liabilities	575,451,000	557,085,000	550,731,000
Regulatory liabilities	357,587,000	349,319,000	346,761,000
Pension and other postretirement benefit liabilities	223,253,000	225,725,000	221,291,000
Derivative instruments	2,546,000	913,000	1,237,000
Other non-current liabilities	144,469,000	142,411,000	143,090,000
Total deferred credits and other non-current liabilities	1,303,306,000	1,275,453,000	1,263,110,000
Commitments and contingencies (see Note 14 and Note 15)
Equity: [Abstract]
Common stock - no par value; authorized 100,000 shares; issued and outstanding 28,644, 27,493, and 28,630 at March 31, 2017 and 2016 and December 31, 2016, respectively	442,647,000	445,187,000	385,232,000
Retained earnings	438,783,000	412,261,000	428,691,000
Accumulated other comprehensive loss	(6,815,000)	(6,951,000)	(6,968,000)
Total equity	874,615,000	850,497,000	806,955,000
Total liabilities and equity	$ 3,070,656,000	$ 3,079,801,000	$ 3,002,033,000