Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Mar. 31, 2013
Related Party Transaction [Line Items]
Credit Support Fees	$ 20	$ 8
Notes Payable	48		48
Loans payable to affiliate	0		0
Notes Receivable	1,250		931
Accounts Receivable	71		22
Accounts Payable	153		192
Subvention Revenues	232	226
Stock-based Compensation Expense	2		2
Operating Leases to Affiliates	7		7
Employee benefits expense	9	7
Investments in affiliates commercial paper	$ 24		$ 2