Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Mar. 31, 2014
Related Party Transaction [Line Items]
Credit Support Fees	$ 22	$ 20	$ 43	$ 40
Notes Payable	33		33		22
Notes Receivable	336		336		1,172
Accounts Receivable	80		80		74
Accounts Payable	149		149		216
Subvention Revenues	296	250	569	482
Stock-based Compensation Expense	2				2
Operating Leases to Affiliates	7		7		7
Employee benefits expense	7	9	13	18
Investments in affiliates' commercial paper	$ 46		$ 46		$ 0