Consolidated Statement of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities: [Abstract]
Net Income	$ 40,310	$ 36,641
Adjustments to reconcile net income to cash provided by operations: [Abstract]
Depreciation and amortization	21,085	20,394
Regulatory amortization of gas reserves	4,107	4,075
Deferred Tax Liabilities, net	20,445	23,353
Qualified defined benefit pension plan expense	1,316	1,311
Contributions to qualified defined benefit pension plans	(3,220)	(2,900)
Deferred environmental (expenditures) recoveries, net	(3,432)	(2,665)
Regulatory disallowance of prior environmental cost deferrals	0	3,273
Amortization of environmental remediation	6,954	5,029
Other	1,695	1,169
Changes in assets and liabilities: [Abstract]
Receivables, net	23,147	22,242
Inventories	5,645	10,115
Taxes Accrued	4,504	7,729
Accounts payable	(13,437)	(14,537)
Interest accrued	4,615	3,798
Deferred gas costs	13,454	8,519
Other, net	17,978	18,592
Net Cash Provided by (Used in) Operating Activities, Continuing Operations	145,166	146,138
Payments to Acquire Property, Plant, and Equipment [Abstract]
Capital expenditures	(38,924)	(30,054)
Other	98	24
Net Cash Provided by (Used in) Investing Activities, Continuing Operations	(38,826)	(30,030)
Financing activities: [Abstract]
Repurchases related to stock-based compensation	(1,943)	(996)
Proceeds from stock options exercised	686	2,995
Change in short-term debt	(53,300)	(105,135)
Cash dividend payments on common stock	(13,456)	(12,823)
Other	(1,209)	(39)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations	(69,222)	(115,998)
(Decrease) Increase in cash and cash equivalents	37,118	110
Cash and cash equivalents, beginning of period	3,521	4,211
Cash and cash equivalents, end of period	40,639	4,321
Supplemental disclosure of cash flow information: [Abstract]
Interest paid, net of capitalization	4,394	5,232
Income Taxes Paid, Net of refunds	$ 3,040	$ (7,900)