Summary of Commercial and Standby Letters of Credit (Detail) (USD $) In Thousands	Sep. 30, 2011
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 735,124
Expires After One Year	119,418
Total Amount Outstanding	854,542
Maximum Amount of Future Payments	854,542
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	686,753
Expires After One Year	115,563
Total Amount Outstanding	802,316
Maximum Amount of Future Payments	802,316
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	40,596
Expires After One Year	3,855
Total Amount Outstanding	44,451
Maximum Amount of Future Payments	44,451
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	7,775
Total Amount Outstanding	7,775
Maximum Amount of Future Payments	$ 7,775