Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating revenues [Abstract]
Operating revenues	$ 88,195	$ 87,501	$ 497,770	$ 501,131
Operating expenses [Abstract]
Cost of gas	33,655	37,570	235,156	241,823
Operations and maintenance	32,636	28,973	99,610	95,543
General taxes	6,954	7,473	23,028	23,726
Depreciation and amortization	18,737	18,281	56,474	54,330
Total operating expenses	91,982	92,297	414,268	415,422
Income (loss) from operations	(3,787)	(4,796)	83,502	85,709
Other income and expense, net	1,300	1,180	3,270	2,272
Interest expense, net	11,347	10,508	33,543	32,163
Income (loss) before income taxes	(13,834)	(14,124)	53,229	55,818
Income tax expense (benefit)	(5,601)	(3,245)	21,697	25,186
Net income (loss)	(8,233)	(10,879)	31,532	30,632
Other comprehensive income [Abstract]
Amortization of non-qualified employee benefit plan liability, net of taxes of $152 and $108 for the three months and $454 and $325 for the nine months ended September 30, 2013 and 2012, respectively	232	167	697	499
Comprehensive income (loss)	$ (8,001)	$ (10,712)	$ 32,229	$ 31,131
Average common shares outstanding [Abstract]
Basic	26,987	26,847	26,962	26,813
Diluted	26,987	26,847	27,013	26,902
Earnings (loss) per share of common stock [Abstract]
Basic	$ (0.31)	$ (0.41)	$ 1.17	$ 1.14
Diluted	$ (0.31)	$ (0.41)	$ 1.17	$ 1.14
Dividends declared per share of common stock	$ 0.455	$ 0.445	$ 1.365	$ 1.335