Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Operating activities [Abstract]
Net income	$ 31,532	$ 30,632
Adjustments to reconcile net income to cash provided by operations [Abstract]
Depreciation and amortization	56,474	54,330
Deferred tax liabilities	22,003	23,965
Non-cash expenses related to qualified defined benefit pension plans	4,256	4,334
Contributions to qualified defined benefit pension plans	(8,900)	(23,500)
Deferred environmental expenditures, net of recoveries	(10,805)	(6,500)
Other	6,016	2,612
Changes in assets and liabilities [Abstract]
Receivables	70,154	106,620
Inventories	(7,817)	1,175
Taxes accrued	3,357	4,780
Accounts payable	(19,860)	(24,888)
Interest accrued	5,190	4,736
Deferred gas costs	(4,159)	(15,406)
Other, net	9,961	15,172
Cash provided by operating activities	157,402	178,062
Investing activities [Abstract]
Capital expenditures	(86,287)	(100,880)
Utility gas reserves	(41,777)	(41,775)
Proceeds from sale of assets	6,580	0
Other	2,116	107
Cash used in investing activities	(119,368)	(142,548)
Financing activities [Abstract]
Common stock issued, net	3,754	4,858
Long-term debt issued	50,000	0
Long-term debt retired	0	(40,000)
Change in short-term debt	(48,950)	34,200
Cash dividend payments on common stock	(36,783)	(35,779)
Other	1,127	1,092
Cash used in financing activities	(30,852)	(35,629)
Increase (decrease) in cash and cash equivalents	7,182	(115)
Cash and cash equivalents, beginning of period	8,923	5,833
Cash and cash equivalents, end of period	16,105	5,718
Supplemental disclosure of cash flow information [Abstract]
Interest paid	28,353	27,427
Income taxes paid	$ 570	$ 2,333