Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities: [Abstract]
Net income (loss)	$ (55,623)	$ 58,895	$ 53,703
Adjustments to reconcile net income to cash provided by operations: [Abstract]
Depreciation and amortization	85,578	82,289	80,923
Regulatory amortization of gas reserves	16,353	15,525	17,991
Deferred Tax Liabilities, net	(52,414)	32,056	26,972
Qualified defined benefit pension plan expense	5,364	5,274	5,697
Contributions to qualified defined benefit pension plans	(19,430)	(14,470)	(14,120)
Deferred environmental (expenditures) recoveries, net	(13,716)	(10,469)	(10,568)
Regulatory disallowance of prior environmental cost deferrals	0	3,287	15,000
Amortization of environmental remediation	15,291	13,298	3,513
Impairment of Long-Lived Assets	192,478	0	0
Other	2,127	3,225	(1,613)
Changes in assets and liabilities: [Abstract]
Receivables, net	3,099	(7,484)	2,373
Inventories	5,571	16,620	6,964
Taxes Accrued	6,734	9,467	(6,541)
Accounts payable	1,424	12,380	(17,175)
Interest accrued	807	93	(206)
Deferred gas costs	17,122	(10,204)	31,918
Other, net	(4,061)	12,365	(10,143)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	(206,704)	(222,147)	(184,688)
Payments to Acquire Property, Plant, and Equipment [Abstract]
Capital expenditures	(213,595)	(139,511)	(118,320)
Other	(577)	2,882	3,022
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(214,172)	(136,629)	(115,298)
Financing activities: [Abstract]
Payments for Repurchase of Common Stock	(2,034)	(1,042)	0
Proceeds from Stock Options Exercised	4,819	8,404	3,875
Common Stock Issued, net	0	52,760	0
Long-term debt issued	100,000	150,000	0
Long-term debt retired	(40,000)	(25,000)	(60,000)
Change in short-term debt	900	(216,735)	35,335
Cash dividend payments on common stock	(53,957)	(51,508)	(49,243)
Other	(2,309)	(3,087)	(4,680)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations	7,419	(86,208)	(74,713)
(Decrease) Increase in cash and cash equivalents	(49)	(690)	(5,323)
Cash and cash equivalents, beginning of period	3,521	4,211	9,534
Cash and cash equivalents, end of period	3,472	3,521	4,211
Supplemental disclosure of cash flow information: [Abstract]
Interest paid, net of capitalization	34,787	36,023	39,634
Income Taxes Paid, Net of refunds	$ 14,780	$ (7,157)	$ 17,306