Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating activities [Abstract]
Net income	$ 59,855	$ 63,898	$ 72,667
Adjustments to reconcile net income to cash provided by operations [Abstract]
Depreciation and amortization	73,017	70,004	65,124
Deferred tax liabilities	42,780	46,877	76,410
Non-cash expenses related to qualified defined benefit pension plans	5,448	7,191	8,009
Contributions to qualified defined benefit pension plans	(23,500)	(22,045)	(10,000)
Deferred environmental expenditures, net of recoveries	(12,503)	25,586	(7,826)
Other	3,990	280	(2,853)
Changes in assets and liabilities [Abstract]
Receivables	22,170	(6,246)	15,830
Inventories	6,761	6,022	572
Taxes accrued	3,334	34,189	(51,524)
Accounts payable	(602)	148	(11,846)
Interest accrued	96	675	(253)
Deferred gas costs	(17,644)	8,565	(26,090)
Other, net	5,636	(1,682)	(1,751)
Cash provided by operating activities	168,838	233,462	126,469
Investing activities [Abstract]
Capital expenditures	(132,029)	(100,534)	(248,505)
Utility gas reserves	(54,085)	(50,597)	0
Restricted cash	0	(3,076)	34,619
Other	1,437	1,142	1,015
Cash used in investing activities	(184,677)	(153,065)	(212,871)
Financing activities [Abstract]
Common stock issued, net	6,758	3,040	4,598
Long-term debt issued	50,000	90,000	0
Long-term debt retired	(40,000)	(10,000)	(35,000)
Change in short-term debt	48,650	(115,835)	155,435
Cash dividend payments on common stock	(48,007)	(46,690)	(44,652)
Other	1,528	1,464	1,046
Cash provided by (used in) financing activities	18,929	(78,021)	81,427
Increase (decrease) in cash and cash equivalents	3,090	2,376	(4,975)
Cash and cash equivalents, beginning of period	5,833	3,457	8,432
Cash and cash equivalents, end of period	8,923	5,833	3,457
Supplemental disclosure of cash flow information [Abstract]
Interest paid	43,061	41,413	41,037
Income taxes paid	$ 2,979	$ 1,756	$ 22,600