Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating activities [Abstract]
Net income	$ 37,639	$ 40,284
Adjustments to reconcile net income to cash provided by operations [Abstract]
Depreciation and amortization	18,807	17,950
Deferred tax liabilities	25,797	26,879
Non-cash expenses related to qualified defined benefit pension plans	1,476	2,007
Contributions to qualified defined benefit pension plans	(1,400)	(13,800)
Deferred environmental expenditures, net of recoveries	(4,482)	(827)
Other	1,836	476
Changes in assets and liabilities [Abstract]
Receivables	5,281	6,378
Inventories	15,598	12,927
Taxes accrued	1,193	5,072
Accounts payable	(13,781)	(26,050)
Interest accrued	4,999	4,791
Deferred gas costs	1,966	23,663
Other, net	11,189	14,304
Cash provided by operating activities	106,118	114,054
Investing activities [Abstract]
Capital expenditures	(22,674)	(20,447)
Utility gas reserves	(12,257)	(17,220)
Other	(1,335)	(68)
Cash used in investing activities	(36,266)	(37,735)
Financing activities [Abstract]
Common stock issued, net	1,115	1,458
Long-term debt retired	0	(40,000)
Change in short-term debt	(59,500)	(27,900)
Cash dividend payments on common stock	(12,248)	(11,913)
Other	195	234
Cash used in financing activities	(70,438)	(78,121)
Decrease in cash and cash equivalents	(586)	(1,802)
Cash and cash equivalents, beginning of period	8,923	5,833
Cash and cash equivalents, end of period	8,337	4,031
Supplemental disclosure of cash flow information [Abstract]
Interest paid	6,128	6,148
Income taxes paid	$ 0	$ 101