Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Operating revenues [Abstract]
Operating revenues	$ 131,714	$ 103,991	$ 409,575	$ 413,630
Operating expenses [Abstract]
Cost of gas	59,142	34,498	201,501	204,253
Operations and maintenance	33,217	32,138	66,974	66,570
General taxes	7,342	7,417	16,074	16,253
Depreciation and amortization	18,930	18,099	37,737	36,049
Total operating expenses	118,631	92,152	322,286	323,125
Income from operations	13,083	11,839	87,289	90,505
Other income and expense, net	1,450	620	1,970	1,092
Interest expense, net	11,069	10,464	22,196	21,655
Income before income taxes	3,464	1,995	67,063	69,942
Income tax expense	1,338	768	27,298	28,431
Net income	2,126	1,227	39,765	41,511
Other comprehensive income [Abstract]
Amortization of non-qualified employee benefit plan liability, net of taxes of $151 and $109 for the three months and $302 and $217 for the six months ended June 30, 2013 and 2012, respectively	232	166	465	332
Comprehensive income	$ 2,358	$ 1,393	$ 40,230	$ 41,843
Average common shares outstanding [Abstract]
Basic	26,958	26,812	26,943	26,797
Diluted	26,999	26,896	26,991	26,879
Earnings per share of common stock [Abstract]
Basic	$ 0.08	$ 0.05	$ 1.48	$ 1.55
Diluted	$ 0.08	$ 0.05	$ 1.47	$ 1.54
Dividends declared per share of common stock	$ 0.455	$ 0.445	$ 0.910	$ 0.890