Summary of Commercial and Standby Letters of Credit (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 817,354
Expires After One Year	71,911
Total Amount Outstanding	889,265	[1]	861,191	[1]
Maximum Amount of Future Payments	889,265
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	696,922
Expires After One Year	60,234
Total Amount Outstanding	757,156
Maximum Amount of Future Payments	757,156
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	63,336
Expires After One Year	11,677
Total Amount Outstanding	75,013
Maximum Amount of Future Payments	75,013
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	57,096
Expires After One Year	0
Total Amount Outstanding	57,096
Maximum Amount of Future Payments	$ 57,096

[1] See below for additional information on our commercial and standby letters of credit.