Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues [Abstract]
Operating revenues	$ 730,607	$ 828,055	$ 792,115
Operating expenses [Abstract]
Cost of gas	355,335	458,508	424,494
Operations and maintenance	129,477	125,417	121,020
General taxes	30,598	29,281	23,872
Depreciation and amortization	73,017	70,004	65,124
Total operating expenses	588,427	683,210	634,510
Income from operations	142,180	144,845	157,605
Other income and expense, net	4,936	4,523	7,102
Interest expense, net	43,157	42,088	42,578
Income before income taxes	103,959	107,280	122,129
Income tax expense	44,104	43,382	49,462
Net income	59,855	63,898	72,667
Other comprehensive income [Abstract]
Change in employee benefit plan liability, net of taxes of $1,339 for 2012, $1,161 for 2011, and $674 for 2010	(2,156)	(1,779)	(1,027)
Amortization of non-qualified employee benefit plan liability, net of taxes of ($434) for 2012, ($383) for 2011, and ($257) for 2010	665	583	391
Comprehensive income	$ 58,364	$ 62,702	$ 72,031
Average common shares outstanding [Abstract]
Basic	26,831	26,687	26,589
Diluted	26,907	26,744	26,657
Earnings per share of common stock [Abstract]
Basic	$ 2.23	$ 2.39	$ 2.73
Diluted	$ 2.22	$ 2.39	$ 2.73
Dividends declared per share of common stock	$ 1.79	$ 1.75	$ 1.68