Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating revenues [Abstract]
Operating revenues	$ 87,199	$ 88,195	$ 513,754	$ 497,770
Operating expenses [Abstract]
Cost of gas	32,227	33,655	245,708	235,156
Operations and maintenance	32,968	32,636	103,085	99,610
General taxes	7,143	6,954	22,508	23,028
Depreciation and amortization	19,938	18,737	59,236	56,474
Total operating expenses	92,276	91,982	430,537	414,268
Income from operations	(5,077)	(3,787)	83,217	83,502
Other income and expense, net	407	1,300	2,052	3,270
Interest expense, net	10,805	11,347	34,024	33,543
Income before income taxes	(15,475)	(13,834)	51,245	53,229
Income tax expense	(6,742)	(5,601)	21,023	21,697
Net income	(8,733)	(8,233)	30,222	31,532
Other comprehensive income [Abstract]
Amortization of non-qualified employee benefit plan liability, net of taxes of $108 and $152 for the three months and $324 and $454 for the nine months ended September 30, 2014 and 2013, respectively	166	232	497	697
Comprehensive income	$ (8,567)	$ (8,001)	$ 30,719	$ 32,229
Average common shares outstanding [Abstract]
Basic	27,189	26,987	27,145	26,962
Diluted	27,189	26,987	27,195	27,013
Earnings per share of common stock [Abstract]
Basic	$ (0.32)	$ (0.31)	$ 1.11	$ 1.17
Diluted	$ (0.32)	$ (0.31)	$ 1.11	$ 1.17
Dividends declared per share of common stock	$ 0.460	$ 0.455	$ 1.380	$ 1.365