Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary of Commercial and Standby Letters of Credit (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 2,014,671
Expires After One Year	57,582
Total Amount Outstanding	2,072,253	$ 1,950,211
Maximum Amount of Future Payments	2,072,253
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	1,906,562
Expires After One Year	41,565
Total Amount Outstanding	1,948,127
Maximum Amount of Future Payments	1,948,127
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	84,402
Expires After One Year	15,666
Total Amount Outstanding	100,068
Maximum Amount of Future Payments	100,068
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	23,707
Expires After One Year	351
Total Amount Outstanding	24,058
Maximum Amount of Future Payments	$ 24,058