Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Operating activities [Abstract]
Net income	$ 39,765	$ 41,511
Adjustments to reconcile net income to cash provided by operations [Abstract]
Depreciation and amortization	37,737	36,049
Deferred tax liabilities	28,401	28,346
Non-cash expenses related to qualified defined benefit pension plans	2,773	4,109
Contributions to qualified defined benefit pension plans	(4,200)	(18,400)
Deferred environmental expenditures, net of recoveries	(2,989)	(3,925)
Other	3,403	1,459
Changes in assets and liabilities [Abstract]
Receivables	63,102	114,117
Inventories	5,190	5,495
Taxes accrued	(1,535)	(1,616)
Accounts payable	(22,155)	(37,854)
Interest accrued	451	(250)
Deferred gas costs	(648)	(11,830)
Other, net	10,847	18,171
Cash provided by operating activities	160,142	175,382
Investing activities [Abstract]
Capital expenditures	(55,055)	(61,552)
Utility gas reserves	(34,397)	(27,060)
Proceeds from sale of assets	6,580	0
Other	1,743	61
Cash used in investing activities	(81,129)	(88,551)
Financing activities [Abstract]
Common stock issued, net	2,355	2,910
Long-term debt retired	0	(40,000)
Change in short-term debt	(54,250)	(28,400)
Cash dividend payments on common stock	(24,509)	(23,839)
Other	682	667
Cash used in financing activities	(75,722)	(88,662)
Increase (decrease) in cash and cash equivalents	3,291	(1,831)
Cash and cash equivalents, beginning of period	8,923	5,833
Cash and cash equivalents, end of period	12,214	4,002
Supplemental disclosure of cash flow information [Abstract]
Interest paid	21,746	21,652
Income taxes paid	$ 0	$ 2,648