Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary of Commercial and Standby Letters of Credit (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Expires In One Year or Less	$ 1,945,524
Expires After One Year	45,399
Total Amount Outstanding	1,990,923	$ 1,950,211
Maximum Amount of Future Payments	1,990,923
Financial standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	1,828,990
Expires After One Year	42,039
Total Amount Outstanding	1,871,029
Maximum Amount of Future Payments	1,871,029
Performance standby letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	107,170
Expires After One Year	2,935
Total Amount Outstanding	110,105
Maximum Amount of Future Payments	110,105
Commercial letters of credit
Guarantor Obligations [Line Items]
Expires In One Year or Less	9,364
Expires After One Year	425
Total Amount Outstanding	9,789
Maximum Amount of Future Payments	$ 9,789