Debt (Details) (USD $)			3 Months Ended				3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended		3 Months Ended			12 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2012 Commercial paper [Member]	Mar. 31, 2012 Commercial paper [Member]	Sep. 30, 2012 Unsecured notes and loans payable [Member]	Mar. 31, 2012 Unsecured notes and loans payable [Member]	Sep. 30, 2012 Unsecured notes and loans payable [Member] Floating rate [Member]	Mar. 31, 2012 Unsecured notes and loans payable [Member] Floating rate [Member]	Sep. 30, 2012 Unsecured notes and loans payable [Member] Fixed rate [Member]	Mar. 31, 2012 Unsecured notes and loans payable [Member] Fixed rate [Member]	Sep. 30, 2012 Secured notes and loans payable [Member]	Mar. 31, 2012 Secured notes and loans payable [Member]	Sep. 30, 2012 Secured notes and loans payable [Member] Floating rate [Member]	Sep. 30, 2011 Secured notes and loans payable [Member] Floating rate [Member]	Sep. 30, 2012 Secured notes and loans payable [Member] Fixed rate [Member]	Mar. 31, 2012 Secured notes and loans payable [Member] Fixed rate [Member]	Sep. 30, 2012 Carrying value adjustment [Member]	Mar. 31, 2012 Carrying value adjustment [Member]
Debt Instrument [Line Items]
Debt	$ 76,674,000,000	$ 73,234,000,000	$ 24,173,000,000	$ 21,247,000,000	$ 42,475,000,000	$ 41,415,000,000	$ 17,600,000,000	$ 16,700,000,000	$ 25,600,000,000	$ 25,500,000,000	$ 9,336,000,000	$ 9,789,000,000					$ 690,000,000	$ 783,000,000
Weighted average contractual interest rate	1.48%	1.70%	0.33%	0.38%	2.32%	2.63%					0.66%	0.67%
Debt denominated in foreign currency	$ 12,000,000,000	$ 15,800,000,000
Contractual interest rate, minimum							0.00%	0.00%	0.50%	0.50%			0.40%	0.40%	0.40%	0.40%
Contractual interest rate, maximum							6.00%	6.00%	9.40%	9.40%			1.90%	1.90%	1.90%	1.90%
Commercial paper average remaining maturity			82
Debt maturity dates											2047