Off-Balance Sheet Arrangements, Guarantees and Other Commitments Summary of Commercial and Standby Letters of Credit (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 1,878,287
Expires After One Year	71,924
Total Amount Outstanding	1,950,211	$ 1,695,856
Maximum Amount of Future Payments	1,950,211
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	1,765,857
Expires After One Year	60,819
Total Amount Outstanding	1,826,676
Maximum Amount of Future Payments	1,826,676
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	94,519
Expires After One Year	11,105
Total Amount Outstanding	105,624
Maximum Amount of Future Payments	105,624
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	17,911
Expires After One Year	0
Total Amount Outstanding	17,911
Maximum Amount of Future Payments	$ 17,911