Commitments And Contingencies (Claims Received By Loan Origination Year) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Apr. 30, 2012	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2010	Apr. 30, 2012	Apr. 30, 2010	Apr. 30, 2009
Commitments And Contingencies [Line Items]
Claims received for loans	$ 543	$ 35	$ 483	$ 31	$ 55	$ 33	$ 21	$ 109	$ 1,887	$ 145	$ 432
2005 [Member]
Commitments And Contingencies [Line Items]
Claims received for loans		4			1		1	6	89	15	62
2006 [Member]
Commitments And Contingencies [Line Items]
Claims received for loans	137	29	130	29	50	29	15	100	844	108	217
2007 [Member]
Commitments And Contingencies [Line Items]
Claims received for loans	$ 406	$ 2	$ 353	$ 2	$ 4	$ 4	$ 5	$ 3	$ 954	$ 22	$ 153