Commitments and Contingencies (Claims Received by Loan Origination Year) (Details) (USD $) In Millions	3 Months Ended												36 Months Ended
	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2010	Apr. 30, 2010	Jan. 31, 2010	Oct. 31, 2009	Jul. 31, 2009	Apr. 30, 2009	Jan. 31, 2009	Oct. 31, 2008	Jul. 31, 2008	Apr. 30, 2011
Claims received for loans	$ 55	$ 33	$ 21	$ 109	$ 45	$ 11	$ 83	$ 6	$ 22	$ 197	$ 41	$ 172	$ 795
2005 [Member]
Claims received for loans	1		1	6			15			1	21	40	85
2006 [Member]
Claims received for loans	50	29	15	100	45	4	57	2	7	111	10	89	519
2007 [Member]
Claims received for loans	$ 4	$ 4	$ 5	$ 3		$ 7	$ 11	$ 4	$ 15	$ 85	$ 10	$ 43	$ 191