Related Party Transactions (Narratives) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2012
Related Party Transaction [Line Items]
Credit Support Fees	$ 18	$ 9	$ 53	$ 25
Notes Payable	51		51		61
Loans payable to affiliate	0		0		2,201
Notes Receivable	1,681		1,681		1,052
Accounts Receivable	24		24		17
Accounts Payable	120		120		58
Subvention Revenues	238	233	700	719
Stock-based Compensation Expense			2		2
Operating Leases to Affiliates	5		5		4
Employee benefits expense	7	0	22	20
Investments in affiliates commercial paper	$ 6		$ 6		$ 0