CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CONSOLIDATED STATEMENTS OF INCOME
OPERATING REVENUES	$ 3,301,804	$ 3,241,379	$ 3,189,199
OPERATING EXPENSES
Fuel and purchased power	994,790	1,009,464	1,046,815
Operations and maintenance	884,769	904,286	870,185
Depreciation and amortization	404,336	427,054	414,479
Taxes other than income taxes	159,323	147,408	135,328
Other expenses	6,831	6,659	7,509
Total	2,450,049	2,494,871	2,474,316
OPERATING INCOME	851,755	746,508	714,883
OTHER INCOME (DEDUCTIONS)
Allowance for equity funds used during construction (Note 1)	22,436	23,707	22,066
Other income (Note 19)	1,606	3,111	6,387
Other expense (Note 19)	(19,842)	(10,451)	(9,921)
Total	4,200	16,367	18,532
INTEREST EXPENSE
Interest charges	214,616	241,995	244,174
Allowance for borrowed funds used during construction (Note 1)	(14,971)	(18,358)	(16,479)
Total	199,645	223,637	227,695
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	656,310	539,238	505,720
INCOME TAXES (Note 4)	237,317	183,604	160,869
INCOME FROM CONTINUING OPERATIONS	418,993	355,634	344,851
INCOME (LOSS) FROM DISCONTINUED OPERATIONS
Net of income tax expense (benefit) of $(3,813), $7,418 and $16,260 (Note 21)	(5,829)	11,306	25,358
NET INCOME	413,164	366,940	370,209
Less: Net income attributable to noncontrolling interests (Note 20)	31,622	27,467	20,156
NET INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS	381,542	339,473	350,053
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING - BASIC (in shares)	109,510,000	109,053,000	106,573,000
WEIGHTED-AVERAGE COMMON SHARES OUTSTANDING - DILUTED (in shares)	110,527,311	109,864,243	107,137,785
EARNINGS PER WEIGHTED - AVERAGE COMMON SHARE OUTSTANDING
Income from continuing operations attributable to common shareholders - basic (in dollars per share)	$ 3.54	$ 3.01	$ 3.05
Net income attributable to common shareholders - basic (in dollars per share)	$ 3.48	$ 3.11	$ 3.28
Income from continuing operations attributable to common shareholders - diluted (in dollars per share)	$ 3.50	$ 2.99	$ 3.03
Net income attributable to common shareholders - diluted (in dollars per share)	$ 3.45	$ 3.09	$ 3.27
DIVIDENDS DECLARED PER SHARE (in dollars per share)	$ 2.67	$ 2.10	$ 2.10
AMOUNTS ATTRIBUTABLE TO COMMON SHAREHOLDERS:
Income from continuing operations, net of tax	387,380	328,110	324,688
Discontinued operations, net of tax	(5,838)	11,363	25,365
Net income attributable to common shareholders	$ 381,542	$ 339,473	$ 350,053