Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary of Commercial and Standby Letters of Credit (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 1,780,099
Expires After One Year	61,286
Total Amount Outstanding	1,841,385	$ 1,695,856
Maximum Amount of Future Payments	1,841,385
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	1,666,751
Expires After One Year	54,244
Total Amount Outstanding	1,720,995
Maximum Amount of Future Payments	1,720,995
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	85,220
Expires After One Year	7,042
Total Amount Outstanding	92,262
Maximum Amount of Future Payments	92,262
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	28,128
Expires After One Year	0
Total Amount Outstanding	28,128
Maximum Amount of Future Payments	$ 28,128