Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013
Related Party Transaction [Line Items]
Credit Support Fees	$ 20	$ 27	$ 40	$ 35
Notes Payable	53		53		48
Loans payable to affiliate	0		0		0
Notes Receivable	1,288		1,288		931
Accounts Receivable	67		67		22
Accounts Payable	148		148		192
Subvention Revenues	250	236	482	462
Stock-based Compensation Expense	2				2
Operating Leases to Affiliates	7		7		7
Employee benefits expense	9	8	18	15
Investments in affiliates commercial paper	$ 5		$ 5		$ 2