Premises and Equipment - Summary of Premises and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 102,454	$ 94,899
Less: Accumulated depreciation	54,471	46,028
Property, plant and equipment, net	47,983	48,871
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,758	2,916
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	6,155	7,391
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	48,573	44,493
Furniture and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 44,968	$ 40,099