Premises and Equipment, Net and Other Assets - Summary of Premises and Equipment by Classification (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Accumulated depreciation	$ (134,022)	$ (123,464)
Premises and equipment, net	261,677	206,275
Buildings [Member]
Property Plant And Equipment [Line Items]
Premises and Equipment, gross	$ 201,606	190,193
Buildings [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated Useful Lives	10 years
Buildings [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated Useful Lives	40 years
Furniture Fixtures and Equipment [Member]
Property Plant And Equipment [Line Items]
Premises and Equipment, gross	$ 87,001	81,225
Furniture Fixtures and Equipment [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated Useful Lives	3 years
Furniture Fixtures and Equipment [Member] | Maximum [Member]
Property Plant And Equipment [Line Items]
Estimated Useful Lives	15 years
Land [Member]
Property Plant And Equipment [Line Items]
Premises and Equipment, gross	$ 45,792	45,561
Construction in Progress [Member]
Property Plant And Equipment [Line Items]
Premises and Equipment, gross	$ 61,300	$ 12,760