Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating activities: [Abstract]
Net income	$ 60,538	$ 58,779	$ 63,044
Adjustments to reconcile net income to cash provided by operations: [Abstract]
Depreciation and amortization	75,905	73,017	70,004
Regulatory amortization of gas reserves	11,089	6,340	1,143
Deferred tax liabilities, net	46,483	42,079	46,319
Non-cash expenses related to qualified defined benefit pension plans	5,666	5,448	7,191
Contributions to qualified defined benefit pension plans	(11,700)	(23,500)	(22,045)
Deferred environmental expenditures, net of recoveries	(16,679)	(12,503)	25,586
Other	(2,580)	(2,350)	(863)
Changes in assets and liabilities: [Abstract]
Receivables	(26,094)	22,170	(6,246)
Inventories	6,933	6,761	6,022
Taxes accrued	286	3,334	34,189
Accounts payable	7,422	(602)	148
Interest accrued	1,150	96	675
Deferred gas costs	(5,245)	(17,644)	8,565
Other, net	23,216	7,413	(270)
Cash provided by operating activities	176,390	168,838	233,462
Investing activities: [Abstract]
Capital expenditures	(138,924)	(132,029)	(100,534)
Utility gas reserves	(54,077)	(54,085)	(50,597)
Proceeds from sale of assets	8,638	0	0
Restricted cash	0	0	(3,076)
Other	2,231	1,437	1,142
Cash used in investing activities	(182,132)	(184,677)	(153,065)
Financing activities: [Abstract]
Common stock issued, net	5,964	6,758	3,040
Long-term debt issued	50,000	50,000	90,000
Long-term debt retired	0	(40,000)	(10,000)
Change in short-term debt	(2,050)	48,650	(115,835)
Cash dividend payments on common stock	(49,204)	(48,007)	(46,690)
Other	1,580	1,528	1,464
Cash provided by (used in) financing activities	6,290	18,929	(78,021)
Increase in cash and cash equivalents	548	3,090	2,376
Cash and cash equivalents, beginning of period	8,923	5,833	3,457
Cash and cash equivalents, end of period	9,471	8,923	5,833
Supplemental disclosure of cash flow information: [Abstract]
Interest paid	44,022	43,061	41,413
Income taxes paid	$ 870	$ 2,979	$ 1,756