Schedule I - Condensed Financial Information - Schedule of Condensed Income Statement (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other Items Net Interest And Other Financial Income [Line Items]
Other income (deductions), net of taxes	$ 6	$ 17	$ 40
Interest expense	(604)	(594)	(609)
NET INCOME FOR COMMON STOCK	1,138	1,051	992
COMPREHENSIVE INCOME FOR COMMON STOCK	1,143	1,033	994
Net Income Per Common Share - Basic	$ 3.88	$ 3.59	$ 3.49
Net Income Per Common Share - Diluted	$ 3.86	$ 3.57	$ 3.47
DIVIDENDS DECLARED PER SHARE OF COMMON STOCK	$ 2.42	$ 2.40	$ 2.38
Average Number Of Shares Outstanding - Basic	292.9	292.6	284.3
Average Number Of Shares Outstanding - Diluted	294.5	294.4	285.9
Parent Company [Member]
Other Items Net Interest And Other Financial Income [Line Items]
Equity in earnings of subsidiaries	1,154	1,064	1,008
Other income (deductions), net of taxes	12	15	12
Interest expense	(28)	(28)	(28)
NET INCOME FOR COMMON STOCK	1,138	1,051	992
COMPREHENSIVE INCOME FOR COMMON STOCK	$ 1,138	$ 1,051	$ 992
Net Income Per Common Share - Basic	$ 3.88	$ 3.59	$ 3.49
Net Income Per Common Share - Diluted	$ 3.86	$ 3.57	$ 3.47
DIVIDENDS DECLARED PER SHARE OF COMMON STOCK	$ 2.42	$ 2.40	$ 2.38
Average Number Of Shares Outstanding - Basic	292.9	292.6	284.3
Average Number Of Shares Outstanding - Diluted	294.5	294.4	285.9