Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012
Related Party Transaction [Line Items]
Credit Support Fees	$ 27	$ 8	$ 35	$ 16
Notes Payable	61		61		61
Loans payable to affiliate	0		0		2,201
Notes Receivable	887		887		1,052
Accounts Receivable	16		16		17
Accounts Payable	134		134		58
Subvention Revenues	236	243	462	486
Stock-based Compensation Expense	2				2
Operating Leases to Affiliates	4		4		4
Employee benefits expense	8	10	15	20
Commercial paper issued by affiliated entity	$ 5		$ 5		$ 0