Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Operating revenues [Abstract]
Operating revenues	$ 133,169	$ 131,714	$ 426,555	$ 409,575
Operating expenses [Abstract]
Cost of gas	58,280	59,142	213,481	201,501
Operations and maintenance	34,731	33,217	70,117	66,974
General taxes	7,183	7,342	15,365	16,074
Depreciation and amortization	19,709	18,930	39,298	37,737
Total operating expenses	119,903	118,631	338,261	322,286
Income from operations	13,266	13,083	88,294	87,289
Other income and expense, net	262	1,450	1,645	1,970
Interest expense, net	11,677	11,069	23,219	22,196
Income before income taxes	1,851	3,464	66,720	67,063
Income tax expense	780	1,338	27,765	27,298
Net income	1,071	2,126	38,955	39,765
Other comprehensive income [Abstract]
Amortization of non-qualified employee benefit plan liability, net of taxes of $108 and $151 for the three months ended and $216 and $302 for the six months ended June 30, 2014 and 2013, respectively	166	232	331	465
Comprehensive income	$ 1,237	$ 2,358	$ 39,286	$ 40,230
Average common shares outstanding [Abstract]
Basic	27,139	26,958	27,116	26,943
Diluted	27,182	26,999	27,158	26,991
Earnings per share of common stock [Abstract]
Basic	$ 0.04	$ 0.08	$ 1.44	$ 1.48
Diluted	$ 0.04	$ 0.08	$ 1.43	$ 1.47
Dividends declared per share of common stock	$ 0.460	$ 0.455	$ 0.920	$ 0.910