Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating revenues [Abstract]
Operating revenues	$ 758,518	$ 730,607	$ 828,055
Operating expenses: [Abstract]
Cost of gas	373,298	355,335	458,508
Operations and maintenance	136,613	129,477	125,417
General taxes	29,956	30,598	29,281
Depreciation and amortization	75,905	73,017	70,004
Total operating expenses	615,772	588,427	683,210
Income from operations	142,746	142,180	144,845
Other income and expense, net	4,669	3,159	3,112
Interest expense, net	45,172	43,157	42,088
Income before income taxes	102,243	102,182	105,869
Income tax expense	41,705	43,403	42,825
Net income	60,538	58,779	63,044
Other comprehensive income: [Abstract]
Change in employee benefit plan liability, net of taxes of ($1,304) for 2013, $1,339 for 2012, and $1,161 for 2011	1,998	(2,156)	(1,779)
Amortization of non-qualified employee benefit plan liability, net of taxes of ($608) for 2013, ($434) for 2012, and ($383) for 2011	935	665	583
Comprehensive income	$ 63,471	$ 57,288	$ 61,848
Average common shares outstanding: [Abstract]
Basic	26,974	26,831	26,687
Diluted	27,027	26,907	26,744
Earnings per share of common stock: [Abstract]
Basic	$ 2.24	$ 2.19	$ 2.36
Diluted	$ 2.24	$ 2.18	$ 2.36
Dividends declared per share of common stock	$ 1.83	$ 1.79	$ 1.75