Summary of Commercial and Standby Letters of Credit (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 755,674
Expires After One Year	86,417
Total Amount Outstanding	842,091	[1]	861,191	[1]
Maximum Amount of Future Payments	842,091
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	684,194
Expires After One Year	76,316
Total Amount Outstanding	760,510
Maximum Amount of Future Payments	760,510
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	57,948
Expires After One Year	10,101
Total Amount Outstanding	68,049
Maximum Amount of Future Payments	68,049
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	13,532
Expires After One Year	0
Total Amount Outstanding	13,532
Maximum Amount of Future Payments	$ 13,532

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