Premises and Equipment - Summary of Premises and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Property, Plant and Equipment [Line Items]
Total	$ 612,969	$ 594,483
Accumulated depreciation	(120,904)	(108,571)
Premises and equipment, net	492,065	485,912	$ 450,253
Land [Member]
Property, Plant and Equipment [Line Items]
Total	79,967	79,231
Buildings [Member]
Property, Plant and Equipment [Line Items]
Total	381,028	365,102
Furniture, Fixtures, and Equipment [Member]
Property, Plant and Equipment [Line Items]
Total	107,526	107,862
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total	$ 44,448	$ 42,288