Summary of Commercial and Standby Letters of Credit (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 774,921
Expires After One Year	86,270
Total Amount Outstanding	861,191
Maximum Amount Of Future Payments	861,191
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	723,710
Expires After One Year	82,346
Total Amount Outstanding	806,056
Maximum Amount Of Future Payments	806,056
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	45,631
Expires After One Year	3,924
Total Amount Outstanding	49,555
Maximum Amount Of Future Payments	49,555
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	5,580
Total Amount Outstanding	5,580
Maximum Amount Of Future Payments	$ 5,580