Consolidated Statements of Income (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Income Statement [Abstract]
Operating revenue	$ 2,651	$ 2,446	$ 5,063	$ 4,678
Fuel	61	83	138	159
Purchased power	822	649	1,437	1,158
Operation and maintenance	866	846	1,717	1,631
Depreciation, decommissioning and amortization	399	356	788	700
Property and other taxes	73	69	156	146
Total operating expenses	2,221	2,003	4,236	3,794
Operating income	430	443	827	884
Interest income	2	2	3	5
Other income	36	39	67	77
Interest expense	(127)	(117)	(249)	(228)
Other expenses	(19)	(13)	(26)	(25)
Income before income taxes	322	354	622	713
Income tax expense	108	128	208	251
Net income	214	226	414	462
Less: Dividends on preferred and preference stock	23	15	41	29
Net income available for common stock	$ 191	$ 211	$ 373	$ 433