Commitments And Contingencies (Claims Received by Loan Origination Year) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										6 Months Ended	12 Months Ended
	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2010	Apr. 30, 2010	Jan. 31, 2010	Oct. 31, 2009	Jul. 31, 2009	Oct. 31, 2011	Apr. 30, 2009
Claims received for loans	$ 483	$ 31	$ 55	$ 33	$ 21	$ 109	$ 45	$ 11	$ 83	$ 6	$ 1,309	$ 432
2005 [Member]
Claims received for loans			1		1	6			15		85	62
2006 [Member]
Claims received for loans	130	29	50	29	15	100	45	4	57	2	678	217
2007 [Member]
Claims received for loans	$ 353	$ 2	$ 4	$ 4	$ 5	$ 3		$ 7	$ 11	$ 4	$ 546	$ 153