Consolidated Statements of Income (USD $) In Thousands, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Operating revenues:
Gross operating revenues	$ 93,313	$ 95,067	$ 577,598	$ 543,961
Less: Cost of sales	43,133	46,359	313,880	281,221
Revenue taxes	2,397	2,497	14,195	13,410
Net operating revenues	47,783	46,211	249,523	249,330
Operating expenses:
Operations and maintenance	28,372	26,913	89,918	85,985
General taxes	7,514	6,659	22,338	17,451
Depreciation and amortization	17,449	16,003	52,304	47,930
Total operating expenses	53,335	49,575	164,560	151,366
Income from operations	(5,552)	(3,364)	84,963	97,964
Other income and expense - net	1,781	1,333	4,117	5,969
Interest expense - net	10,241	10,632	30,956	31,738
Income before income taxes	(14,012)	(12,663)	58,124	72,195
Income tax expense	(5,700)	(5,243)	23,470	29,119
Net income	$ (8,312)	$ (7,420)	$ 34,654	$ 43,076
Average common shares outstanding:
Basic	26,686	26,606	26,676	26,571
Diluted	26,686	26,606	26,730	26,641
Earnings per share of common stock [Abstract]
Earnings per share of common stock - basic	$ (0.31)	$ (0.28)	$ 1.30	$ 1.62
Earnings per share of common stock - diluted	$ (0.31)	$ (0.28)	$ 1.30	$ 1.62
Dividends declared per share of common stock	$ 0.435	$ 0.415	$ 1.305	$ 1.245