Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Operating revenues [Abstract]
Gross operating revenues	$ 89,756	$ 93,313	$ 513,819	$ 577,598
Cost of sales	37,586	43,133	241,869	313,880
Revenue taxes	2,255	2,397	12,688	14,195
Net operating revenues	49,915	47,783	259,262	249,523
Operating expenses [Abstract]
Operations and maintenance	28,957	28,372	95,497	89,918
General taxes	7,473	7,514	23,726	22,338
Depreciation and amortization	18,281	17,449	54,330	52,304
Total operating expenses	54,711	53,335	173,553	164,560
Income (loss) from operations	(4,796)	(5,552)	85,709	84,963
Other income and expense - net	1,710	1,781	3,636	4,117
Interest expense - net	10,508	10,241	32,163	30,956
Income (loss) before income taxes	(13,594)	(14,012)	57,182	58,124
Income tax expense (benefit)	(3,036)	(5,700)	25,724	23,470
Net income (loss)	(10,558)	(8,312)	31,458	34,654
Other comprehensive income
Amortization of non-qualified employee benefit plan liability, net of taxes of $108 and $95 for the three months and $325 and $287 for the nine months ended September 30, 2012 and 2011, respectively	167	146	499	438
Comprehensive income (loss)	$ (10,391)	$ (8,166)	$ 31,957	$ 35,092
Average common shares outstanding [Abstract]
Basic	26,847	26,686	26,813	26,676
Diluted	26,847	26,686	26,902	26,730
Earnings (loss) per share of common stock
Basic	$ (0.39)	$ (0.31)	$ 1.17	$ 1.30
Diluted	$ (0.39)	$ (0.31)	$ 1.17	$ 1.30
Dividends declared per share of common stock	$ 0.445	$ 0.435	$ 1.335	$ 1.305