Consolidated Statements of Income and Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operating revenues:
Gross operating revenues	$ 848,796	$ 812,106	$ 1,012,711
Less: Cost of sales	458,622	424,534	611,168
Revenue taxes	20,741	19,991	24,656
Net operating revenues	369,433	367,581	376,887
Operating expenses:
Operations and maintenance	125,303	120,980	127,104
General taxes	29,281	23,872	28,253
Depreciation and amortization	70,004	65,124	62,814
Total operating expenses	224,588	209,976	218,171
Income from operations	144,845	157,605	158,716
Other income and expense - net	4,523	7,102	3,714
Interest expense - net	42,088	42,578	40,637
Income before income taxes	107,280	122,129	121,793
Income tax expense	43,382	49,462	46,671
Net income	63,898	72,667	75,122
Comprehensive Income [Abstract]
Change in non-qualified employee benefit plan liability, net of tax	(1,779)	(1,027)	(1,936)
Amortization of non-qualified employee benefit plan liabiliy, net of tax	583	391	354
Other Comprehensive Income (Loss), Net of Tax	$ 62,702	$ 72,031	$ 73,540
Average common shares outstanding:
Basic	26,687	26,589	26,511
Diluted	26,744	26,657	26,576
Earnings per share of common stock [Abstract]
Earnings per share of common stock - basic	$ 2.39	$ 2.73	$ 2.83
Earnings per share of common stock - diluted	$ 2.39	$ 2.73	$ 2.83
Dividends declared per share of common stock	$ 1.75	$ 1.68	$ 1.60