Liquidity Facilities and Letters of Credit (Details) (USD $) In Billions, unless otherwise specified	1 Months Ended	12 Months Ended			12 Months Ended			1 Months Ended		12 Months Ended			12 Months Ended			1 Months Ended		12 Months Ended			12 Months Ended			1 Months Ended	12 Months Ended				12 Months Ended					12 Months Ended			12 Months Ended
	Feb. 28, 2013 364 Day Credit Agreement [Member]	Mar. 31, 2012 364 Day Credit Agreement [Member]	Mar. 31, 2013 364 Day Credit Agreement [Member] 2013 [Member]	Mar. 31, 2012 364 Day Credit Agreement [Member] 2013 [Member]	Mar. 31, 2012 364 Day Credit Agreement [Member] 2013 [Member] Refinancing of Debt [Member]	Mar. 31, 2013 364 Day Credit Agreement [Member] 2014 [Member]	Mar. 31, 2012 364 Day Credit Agreement [Member] 2014 [Member]	Feb. 28, 2013 364 Day Credit Agreement [Member] 2014 [Member] Refinancing of Debt [Member]	Feb. 28, 2013 Three Year Agreement [Member]	Mar. 31, 2012 Three Year Agreement [Member]	Mar. 31, 2013 Three Year Agreement [Member] 2014 [Member]	Mar. 31, 2012 Three Year Agreement [Member] 2014 [Member]	Mar. 31, 2012 Three Year Agreement [Member] 2014 [Member] Refinancing of Debt [Member]	Mar. 31, 2013 Three Year Agreement [Member] 2016 [Member]	Mar. 31, 2012 Three Year Agreement [Member] 2016 [Member]	Feb. 28, 2013 Three Year Agreement [Member] 2016 [Member] Refinancing of Debt [Member]	Feb. 28, 2013 Five Year Agreement [Member]	Mar. 31, 2012 Five Year Agreement [Member]	Mar. 31, 2013 Five Year Agreement [Member] 2016 [Member]	Mar. 31, 2012 Five Year Agreement [Member] 2016 [Member]	Mar. 31, 2012 Five Year Agreement [Member] 2016 [Member] Refinancing of Debt [Member]	Mar. 31, 2013 Five Year Agreement [Member] 2018 [Member]	Mar. 31, 2012 Five Year Agreement [Member] 2018 [Member]	Feb. 28, 2013 Five Year Agreement [Member] 2018 [Member] Refinancing of Debt [Member]	Mar. 31, 2013 Committed Revolving Asset-backed Commercial Paper Facility [Member]	Mar. 31, 2013 Other Credit Agreements [Member] Committed Bank Credit [Member] Group A [Member] Refinancing of Debt [Member]	Mar. 31, 2013 Other Credit Agreements [Member] 2014 [Member] Group A [Member]	Mar. 31, 2012 Other Credit Agreements [Member] 2014 [Member] Group A [Member]	Mar. 31, 2013 Other Credit Agreements [Member] 2014 [Member] Committed Bank Credit [Member] Group A [Member] Refinancing of Debt [Member]	Mar. 31, 2013 Other Credit Agreements [Member] 2014 [Member] Uncommitted Bank Credit [Member]	Mar. 31, 2012 Other Credit Agreements [Member] 2014 [Member] Uncommitted Bank Credit [Member]	Mar. 31, 2013 Other Credit Agreements [Member] 2015 [Member] Group A [Member]	Mar. 31, 2012 Other Credit Agreements [Member] 2015 [Member] Group A [Member]	Mar. 31, 2013 Other Credit Agreements [Member] 2015 [Member] Committed Bank Credit [Member] Group A [Member] Refinancing of Debt [Member]	Mar. 31, 2013 Other Credit Agreements [Member] 2016 [Member] Group A [Member]	Mar. 31, 2012 Other Credit Agreements [Member] 2016 [Member] Group A [Member]	Mar. 31, 2013 Other Credit Agreements [Member] 2016 [Member] Committed Bank Credit [Member] Group A [Member] Refinancing of Debt [Member]
Line of Credit Facility [Line Items]
Initiation Date	Feb. 01, 2013	Mar. 31, 2012							Feb. 01, 2013	Mar. 31, 2012							Feb. 01, 2013	Mar. 31, 2012
Maximum Borrowing Capacity	$ 3.8	$ 5.0							$ 3.8	$ 5.0							$ 3.8	$ 3.0							$ 3.0	$ 5.5			$ 3.1	$ 0.5				$ 0.9			$ 1.5
Expiration Date	Feb. 01, 2013				Mar. 31, 2013			Mar. 31, 2014	Feb. 01, 2013				Mar. 31, 2014			Mar. 31, 2016	Feb. 01, 2013				Mar. 31, 2016			Mar. 31, 2018	Jan. 01, 2013				Mar. 31, 2014	Mar. 31, 2014				Mar. 31, 2015			Mar. 31, 2016
Credit facilities amount outstanding			$ 0	$ 0		$ 0	$ 0				$ 0	$ 0		$ 0	$ 0				$ 0	$ 0		$ 0	$ 0				$ 0	$ 0		$ 0	$ 0	$ 0	$ 0		$ 0	$ 0