Consolidated Statement of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities [Abstract]
Net income	$ 30,683	$ 38,955
Adjustments to reconcile net income to cash provided by operations [Abstract]
Depreciation and amortization	40,341	39,298
Regulatory amortization of gas reserves	10,023	8,680
Deferred tax liabilities, net	6,886	989
Non-cash expenses related to qualified defined benefit pension plans	3,032	2,540
Contributions to qualified defined benefit pension plans	(5,810)	(6,000)
Deferred environmental recoveries, net of (expenditures)	(5,659)	92,104
Non-cash regulatory disallowance of prior environmental cost deferrals	(15,000)	0
non-cash interest income deferred environmental expenses	5,322	0
Other	418	1,010
Changes in assets and liabilities [Abstract]
Receivables	85,121	89,951
Inventories	1,321	(139)
Taxes accrued	(249)	8,447
Accounts payable	(37,532)	(24,472)
Interest accrued	(157)	(50)
Deferred gas costs	21,718	(18,812)
Other, net	7,670	744
Cash provided by operating activities	167,484	233,245
Investing activities [Abstract]
Capital expenditures	(58,072)	(52,489)
Utility gas reserves	(1,945)	(18,632)
Increase (Decrease) in Restricted Cash	(1,500)	1,000
Other	201	(1,043)
Cash used in investing activities	(61,316)	(71,164)
Financing activities [Abstract]
Common stock issued, net	812	3,733
Repayments of Long-term Debt	(40,000)	(20,000)
Change in short-term debt	(44,400)	(114,000)
Cash dividend payments on common stock	(25,398)	(24,938)
Other	(2,250)	893
Cash used in financing activities	(111,236)	(154,312)
Increase (decrease) in cash and cash equivalents	(5,068)	7,769
Cash and cash equivalents, beginning of period	9,534	9,471
Cash and cash equivalents, end of period	4,466	17,240
Supplemental disclosure of cash flow information [Abstract]
Interest paid	19,615	23,270
Income Taxes Paid, Net	$ 4,625	$ 14,945