Summary Of Significant Accounting Policies (Earnings Per Common Share) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2012	Jun. 30, 2011
Net income for common stock	$ 214	$ 277	$ 165	$ 311	$ 491	$ 477
Weighted average common shares outstanding - Basic	292.9		292.7		292.9	292.3
Add: Incremental shares attributable to effect of potentially dilutive securities	1.5		1.6		1.5	1.6
Adjusted weighted average common shares outstanding - Diluted	294.4		294.3		294.4	293.9
Net income for common stock per common share - basic	$ 0.73		$ 0.57		$ 1.68	$ 1.63
Net income for common stock per common share - diluted	$ 0.73		$ 0.56		$ 1.67	$ 1.62