Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities: [Abstract]
Net Income	$ 58,895	$ 53,703	$ 58,692
Adjustments to reconcile net income to cash provided by operations: [Abstract]
Depreciation and amortization	82,289	80,923	79,193
Regulatory amortization of gas reserves	15,525	17,991	19,335
Deferred Tax Liabilities, net	32,056	26,972	24,772
Qualified defined benefit pension plan expense	5,274	5,697	4,984
Contributions to qualified defined benefit pension plans	(14,470)	(14,120)	(10,500)
Deferred environmental (expenditures) recoveries, net	(10,469)	(10,568)	88,849
Regulatory disallowance of prior environmental cost deferrals	3,287	15,000	0
Interest income on deferred environmental expenses	0	(5,322)	0
Amortization of environmental remediation	13,298	3,513	0
Other	3,225	3,709	1,853
Changes in assets and liabilities: [Abstract]
Receivables, net	(7,484)	2,373	14,948
Inventories	16,620	6,964	(17,163)
Taxes Accrued	9,467	(6,541)	1,709
Accounts payable	12,380	(17,175)	(2,020)
Interest accrued	93	(206)	(1,024)
Deferred gas costs	(10,204)	31,918	(23,114)
Other, net	12,365	(10,143)	(24,857)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	(222,147)	(184,688)	(215,657)
Payments to Acquire Property, Plant, and Equipment [Abstract]
Capital expenditures	(139,511)	(118,320)	(120,092)
Utility gas reserves	0	(1,549)	(26,798)
Proceeds from sale of assets	521	410	175
Restricted cash	0	3,000	1,000
Other	2,361	1,161	1,392
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(136,629)	(115,298)	(144,323)
Financing activities: [Abstract]
Common Stock Issued, net	60,122	3,875	8,986
Long-term debt issued	150,000	0	0
Long-term debt retired	(25,000)	(60,000)	(80,000)
Change in short-term debt	(216,735)	35,335	46,500
Cash dividend payments on common stock	(51,508)	(49,243)	(50,093)
Other	(3,087)	(4,680)	3,336
Net Cash Provided by (Used in) Financing Activities, Continuing Operations	(86,208)	(74,713)	(71,271)
(Decrease) Increase in cash and cash equivalents	(690)	(5,323)	63
Cash and cash equivalents, beginning of period	4,211	9,534	9,471
Cash and cash equivalents, end of period	3,521	4,211	9,534
Supplemental disclosure of cash flow information: [Abstract]
Interest paid, net of capitalization	36,023	39,634	42,602
Income Taxes Paid, Net of refunds	$ (7,157)	$ 17,306	$ 19,445