Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Mar. 31, 2014
Related Party Transaction [Line Items]
Credit Support Fees	$ 23	$ 21	$ 66	$ 61
Notes Payable	28		28		22
Notes Receivable	821		821		1,172
Accounts Receivable	85		85		74
Accounts Payable	116		116		216
Subvention Revenues	315	251	884	733
Stock-based Compensation Expense	2				2
Operating Leases to Affiliates	7		7		7
Employee benefits expense	$ 5	$ 9	$ 18	$ 27