Consolidated Statement of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating activities [Abstract]
Net income	$ 37,884	$ 37,639
Adjustments to reconcile net income to cash provided by operations [Abstract]
Depreciation and amortization	19,589	18,807
Regulatory amortization of gas reserves	2,981	2,381
Deferred tax liabilities, net	205	25,797
Non-cash expenses related to qualified defined benefit pension plans	1,278	1,476
Contributions to qualified defined benefit pension plans	(2,800)	(1,400)
Deferred environmental recoveries, net of expenditures	83,252	(4,482)
Other	603	(545)
Changes in assets and liabilities [Abstract]
Receivables	23,216	5,281
Inventories	27,200	15,598
Taxes accrued	26,824	1,193
Accounts payable	(1,671)	(13,781)
Interest accrued	4,041	4,999
Deferred gas costs	(14,049)	1,966
Other, net	11,579	11,189
Cash provided by operating activities	220,132	106,118
Investing activities [Abstract]
Capital expenditures	(25,588)	(22,674)
Utility gas reserves	(19,681)	(12,257)
Other	(191)	(1,335)
Cash used in investing activities	(45,460)	(36,266)
Financing activities [Abstract]
Common stock issued, net	1,400	1,115
Change in short-term debt	(155,600)	(59,500)
Cash dividend payments on common stock	(12,456)	(12,248)
Other	442	195
Cash used in financing activities	(166,214)	(70,438)
Increase (decrease) in cash and cash equivalents	8,458	(586)
Cash and cash equivalents, beginning of period	9,471	8,923
Cash and cash equivalents, end of period	17,929	8,337
Supplemental disclosure of cash flow information [Abstract]
Interest paid	7,502	6,128
Income taxes paid	$ 0	$ 0