Debt (Detail) (USD $) In Millions, unless otherwise specified					9 Months Ended					3 Months Ended
	Sep. 30, 2012	Sep. 30, 2012 Pollution Control Bonds Series 1996 C, E, F, And 1997 B [Member]	Sep. 30, 2012 Pollution Control Bonds Series 2009 A-D [Member]	Sep. 30, 2012 Energy Recovery Bonds [Member]	Sep. 30, 2012 Floating Rate Senior Notes [Member]	Sep. 30, 2012 Minimum [Member] Pollution Control Bonds Series 1996 C, E, F, And 1997 B [Member]	Sep. 30, 2012 Minimum [Member] Pollution Control Bonds Series 2009 A-D [Member]	Sep. 30, 2012 Maximum [Member] Pollution Control Bonds Series 1996 C, E, F, And 1997 B [Member]	Sep. 30, 2012 Maximum [Member] Pollution Control Bonds Series 2009 A-D [Member]	Sep. 30, 2012 Utility [Member]	Apr. 02, 2012 Utility [Member]	Apr. 16, 2012 Utility [Member] Due April 12 2042 [Member]	Aug. 16, 2012 Utility [Member] Due August 15 2042 [Member]	Aug. 16, 2012 Utility [Member] Due August 15 2022 [Member]
Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 300									$ 3,000
Letters of credit outstanding										330
Senior Notes												400	350	400
Debt instrument, interest rate						0.16%	0.17%	0.21%	0.20%			4.45%	3.75%	2.45%
Senior notes maturity period					2012-11-20					2042-04-15
Debt instrument, face amount		614	309		250
Repurchase of principal amount											50
Commercial paper outstanding										145
Interest rate for the Floating Rate Senior Notes					0.88%
ERB, principal outstanding				$ 110