Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2013
Related Party Transaction [Line Items]
Credit Support Fees	$ 21	$ 18	$ 61	$ 53
Notes Payable	24		24		48
Notes Receivable	1,309		1,309		931
Accounts Receivable	78		78		22
Accounts Payable	51		51		192
Subvention Revenues	251	238	733	700
Stock-based Compensation Expense	2				2
Operating Leases to Affiliates	7		7		7
Employee benefits expense	9	7	27	22
Investments in affiliates commercial paper	$ 0		$ 0		$ 2