Consolidated Statements of Comprehensive Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Operating revenues:
Gross operating revenues	$ 317,494	$ 323,088
Less: Cost of sales	169,771	180,625
Revenue taxes	7,855	7,955
Net operating revenues	139,868	134,508
Operating expenses:
Operations and maintenance	34,416	31,172
General taxes	8,836	8,165
Depreciation and amortization	17,950	17,309
Total operating expenses	61,202	56,646
Income from operations	78,666	77,862
Other income and expense - net	1,005	1,214
Interest expense - net	11,191	10,449
Income before income taxes	68,480	68,627
Income tax expense	27,873	27,854
Net income	40,607	40,773
Comprehensive Income [Abstract]
Other Comprehensive Income (Loss), Amortization, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost Recognized in Net Periodic Pension Cost, Net of Tax	166	146
Other Comprehensive Income (Loss), Net of Tax	$ 40,773	$ 40,919
Average common shares outstanding:
Basic	26,781	26,670
Diluted	26,862	26,724
Earnings per share of common stock [Abstract]
Earnings per share of common stock - basic	$ 1.52	$ 1.53
Earnings per share of common stock - diluted	$ 1.51	$ 1.53
Dividends declared per share of common stock	$ 0.445	$ 0.435