Changes in the Corporation's liability of estimated losses from credit recourses agreements (Detail) (Financial Guarantee Member, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Financial Guarantee Member
Guarantor Obligations [Line Items]
Balance as of beginning of period	$ 55,783	$ 55,327	$ 58,659	$ 53,729
Valuation Allowances And Reserves Charged To Other Accounts	0	0	0	0
Provision	5,576	10,285	15,138	30,109
Valuation Allowances And Reserves Deductions	(5,068)	(10,055)	(17,506)	(28,281)
Balance as of end of period	$ 56,291	$ 55,557	$ 56,291	$ 55,557