Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary of Commercial and Standby Letters of Credit (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 1,963,685
Expires After One Year	56,446
Total Amount Outstanding	2,020,131	$ 1,950,211
Maximum Amount of Future Payments	2,020,131
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	1,850,099
Expires After One Year	43,050
Total Amount Outstanding	1,893,149
Maximum Amount of Future Payments	1,893,149
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	99,750
Expires After One Year	13,345
Total Amount Outstanding	113,095
Maximum Amount of Future Payments	113,095
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	13,836
Expires After One Year	51
Total Amount Outstanding	13,887
Maximum Amount of Future Payments	$ 13,887