Consolidated Statement of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities: [Abstract]
Net Income	$ 53,703	$ 58,692	$ 60,538
Adjustments to reconcile net income to cash provided by operations: [Abstract]
Depreciation and amortization	80,923	79,193	75,905
Regulatory amortization of gas reserves	17,991	19,335	11,089
Deferred Tax Liabilities, net	26,972	24,772	46,483
Qualified defined benefit pension plan expense	5,697	4,984	5,666
Contributions to qualified defined benefit pension plans	(14,120)	(10,500)	(11,700)
Deferred environmental (expenditures) recoveries, net of recoveries	(10,568)	88,849	(16,679)
Regulatory disallowance of prior environmental cost deferrals	15,000	0	0
Interest income on deferred environmental expenses	(5,322)	0	0
Amortization of environmental remediation	3,513	0	0
Other	3,709	1,853	(2,580)
Changes in assets and liabilities: [Abstract]
Receivables, net	2,373	14,948	(26,094)
Inventories	6,964	(17,163)	6,933
Taxes Accrued	(6,541)	1,709	286
Accounts payable	(17,175)	(2,020)	7,422
Interest accrued	(206)	(1,024)	1,150
Deferred gas costs	31,918	(23,114)	(5,245)
Other, net	(10,143)	(24,857)	23,216
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	184,688	215,657	176,390
Payments to Acquire Property, Plant, and Equipment [Abstract]
Capital expenditures	(118,320)	(120,092)	(138,924)
Utility gas reserves	(1,549)	(26,798)	(54,077)
Proceeds from sale of assets	410	175	8,638
Restricted cash	3,000	1,000	0
Other	1,161	1,392	2,231
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(115,298)	(144,323)	(182,132)
Financing activities: [Abstract]
Common Stock Issued, net	3,875	8,986	5,964
Long-term debt issued	0	0	50,000
Long-term debt retired	(60,000)	(80,000)	0
Change in short-term debt	35,335	46,500	(2,050)
Cash dividend payments on common stock	(49,243)	(50,093)	(49,204)
Other	(4,680)	3,336	1,580
Net Cash Provided by (Used in) Financing Activities, Continuing Operations	(74,713)	(71,271)	6,290
Increase in cash and cash equivalents	(5,323)	63	548
Cash and cash equivalents, beginning of period	9,534	9,471	8,923
Cash and cash equivalents, end of period	4,211	9,534	9,471
Supplemental disclosure of cash flow information: [Abstract]
Interest paid, net of capitalization	39,634	42,602	44,022
Income Taxes Paid, Net of refunds	$ 17,306	$ 19,445	$ 870