Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating revenues [Abstract]
Operating revenues	$ 93,128	$ 87,199	$ 493,073	$ 513,754
Operating expenses [Abstract]
Cost of Revenue	35,856	32,227	223,737	245,708
Operations and maintenance	32,031	32,968	121,458	103,085
General taxes	6,772	7,143	23,153	22,508
Depreciation and amortization	20,342	19,938	60,683	59,236
Total operating expenses	95,001	92,276	429,031	430,537
Income (loss) from operations	(1,873)	(5,077)	64,042	83,217
Other income and expense, net	746	407	6,930	2,052
Interest expense, net	10,111	10,805	31,030	34,024
Income (loss) before income taxes	(11,238)	(15,475)	39,942	51,245
Income tax expense	(4,553)	(6,742)	15,944	21,023
Net income (loss)	(6,685)	(8,733)	23,998	30,222
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 $650 and $324 for the nine months ended September 30, 2015 and 2014, respectively	332	166	995	497
Comprehensive income	$ (6,353)	$ (8,567)	$ 24,993	$ 30,719
Average common shares outstanding [Abstract]
Basic	27,363	27,189	27,336	27,145
Diluted	27,363	27,189	27,399	27,195
Earnings per share of common stock [Abstract]
Basic	$ (0.24)	$ (0.32)	$ 0.88	$ 1.11
Diluted	(0.24)	(0.32)	0.88	1.11
Dividends declared per share of common stock	$ 0.465	$ 0.460	$ 1.395	$ 1.380