Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Operating revenues [Abstract]
Operating revenues	$ 277,861	$ 309,639
Operating expenses [Abstract]
Cost of gas	142,359	169,755
Operations and maintenance	33,757	34,432
General taxes	8,732	8,836
Depreciation and amortization	18,807	17,950
Total operating expenses	203,655	230,973
Income from operations	74,206	78,666
Other income and expense, net	520	472
Interest expense, net	11,127	11,191
Income before income taxes	63,599	67,947
Income tax expense	25,960	27,663
Net income	37,639	40,284
Other comprehensive income [Abstract]
Amortization of non-qualified employee benefit plan liability, net of taxes of $151 for 2013 and $108 for 2012	233	166
Comprehensive income	$ 37,872	$ 40,450
Average common shares outstanding [Abstract]
Basic	26,929	26,781
Diluted	26,973	26,862
Earnings per share of common stock [Abstract]
Basic	$ 1.40	$ 1.50
Diluted	$ 1.40	$ 1.50
Dividends declared per share of common stock	$ 0.455	$ 0.445