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Debt (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 3 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Pollution Control Bonds Series 1996 C, E, F, And 1997 B [Member]
Jun. 30, 2013
Pollution Control Bonds Series 2009 A-D [Member]
Jun. 30, 2013
Minimum [Member]
Pollution Control Bonds Series 1996 C, E, F, And 1997 B [Member]
Jun. 30, 2013
Minimum [Member]
Pollution Control Bonds Series 2009 A-D [Member]
Jun. 30, 2013
Maximum [Member]
Pollution Control Bonds Series 1996 C, E, F, And 1997 B [Member]
Jun. 30, 2013
Maximum [Member]
Pollution Control Bonds Series 2009 A-D [Member]
Jun. 30, 2013
Utility [Member]
Jun. 14, 2013
Utility [Member]
Due June 15 2023 [Member]
Jun. 14, 2013
Utility [Member]
Due June 15 2043 [Member]
Jun. 14, 2013
Utility [Member]
Due March 1 2014 [Member]
Debt [Line Items]                      
Line of Credit termination date Apr. 01, 2018             Apr. 01, 2018      
Line of credit facility, maximum borrowing capacity $ 300             $ 3,000      
Letters of credit outstanding               91      
Debt instrument, interest rate       0.04% 0.01% 0.06% 0.05%   3.25% 4.60% 4.80%
Debt instrument, face amount   614 309                
Senior Notes                 375 375 1,000
Secured Debt Repurchase Agreements                     461
Commercial paper outstanding               692      
Short term borrowing outstanding 260                    
Premiums and accrued interest                     $ 21