Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues [Abstract]
Operating revenues	$ 754,037	$ 758,518	$ 730,607
Operating expenses: [Abstract]
Cost of gas	365,490	373,298	355,335
Operations and maintenance	136,982	136,613	129,477
General taxes	29,407	29,956	30,598
Depreciation and amortization	79,193	75,905	73,017
Total operating expenses	611,072	615,772	588,427
Income from operations	142,965	142,746	142,180
Other income and expense, net	1,933	4,669	3,159
Interest expense, net	44,563	45,172	43,157
Income before income taxes	100,335	102,243	102,182
Income tax expense	41,643	41,705	43,403
Net income	58,692	60,538	58,779
Other comprehensive income: [Abstract]
Change in employee benefit plan liability, net of taxes of $2,857 for 2014, ($1,304) for 2013, and $1,339 for 2012	(4,364)	1,998	(2,156)
Amortization of non-qualified employee benefit plan liability, net of taxes of ($438) for 2014, ($608) for 2013, and ($434) for 2012	646	935	665
Comprehensive income	$ 54,974	$ 63,471	$ 57,288
Average common shares outstanding: [Abstract]
Basic	27,164	26,974	26,831
Diluted	27,223	27,027	26,907
Earnings per share of common stock: [Abstract]
Basic	$ 2.16	$ 2.24	$ 2.19
Diluted	$ 2.16	$ 2.24	$ 2.18
Dividends declared per share of common stock	$ 1.85	$ 1.83	$ 1.79