Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Operating revenues [Abstract]
Operating revenues	$ 138,280	$ 133,169	$ 399,945	$ 426,555
Operating expenses [Abstract]
Cost of Revenue	62,176	58,280	187,881	213,481
Operations and maintenance	35,311	34,731	89,427	70,117
General taxes	7,649	7,183	16,381	15,365
Depreciation and amortization	20,230	19,709	40,341	39,298
Total operating expenses	125,366	119,903	334,030	338,261
Income from operations	12,914	13,266	65,915	88,294
Other income and expense, net	1,135	262	6,184	1,645
Interest expense, net	10,438	11,677	20,919	23,219
Income before income taxes	3,611	1,851	51,180	66,720
Income tax expense	1,414	780	20,497	27,765
Net income	2,197	1,071	30,683	38,955
Other comprehensive income [Abstract]
Amortization of non-qualified employee benefit plan liability, net of taxes of $217 and $108 for the three months ended and $433 and $216 for the six months ended June 30, 2015 and 2014, respectively	331	166	663	331
Comprehensive income	$ 2,528	$ 1,237	$ 31,346	$ 39,286
Average common shares outstanding [Abstract]
Basic	27,343	27,139	27,322	27,116
Diluted	27,388	27,182	27,378	27,158
Earnings per share of common stock [Abstract]
Basic	$ 0.08	$ 0.04	$ 1.12	$ 1.44
Diluted	0.08	0.04	1.12	1.43
Dividends declared per share of common stock	$ 0.465	$ 0.460	$ 0.930	$ 0.920