Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating revenues [Abstract]
Operating revenues	$ 293,386	$ 277,861
Operating expenses [Abstract]
Cost of gas	155,201	142,359
Operations and maintenance	35,386	33,757
General taxes	8,182	8,732
Depreciation and amortization	19,589	18,807
Total operating expenses	218,358	203,655
Income from operations	75,028	74,206
Other income and expense, net	1,383	520
Interest expense, net	11,542	11,127
Income before income taxes	64,869	63,599
Income tax expense	26,985	25,960
Net income	37,884	37,639
Other comprehensive income [Abstract]
Amortization of non-qualified employee benefit plan liability, net of taxes of $109 and $151 for the three months ended March 31, 2014 and 2013, respectively	165	233
Comprehensive income	$ 38,049	$ 37,872
Average common shares outstanding [Abstract]
Basic	27,094	26,929
Diluted	27,126	26,973
Earnings per share of common stock [Abstract]
Basic	$ 1.40	$ 1.40
Diluted	$ 1.40	$ 1.40
Dividends declared per share of common stock	$ 0.460	$ 0.455