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			54 Months Ended
	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2010	Oct. 31, 2012	Apr. 30, 2012	Apr. 30, 2010	Apr. 30, 2009	Oct. 31, 2012
Commitments And Contingencies [Line Items]
Claims received for loans	$ 10	$ 142	$ 543	$ 35	$ 483	$ 31	$ 55	$ 33	$ 21	$ 109	$ 152	$ 1,100	$ 145	$ 432	$ 2,039
2005 [Member]
Commitments And Contingencies [Line Items]
Claims received for loans		18		4			1		1	6			15	62	107
2006 [Member]
Commitments And Contingencies [Line Items]
Claims received for loans	7	123	137	29	130	29	50	29	15	100			108	217	974
2007 [Member]
Commitments And Contingencies [Line Items]
Claims received for loans	$ 3	$ 1	$ 406	$ 2	$ 353	$ 2	$ 4	$ 4	$ 5	$ 3			$ 22	$ 153	$ 958