Premises and Equipment, Net and Other Assets - Summary of premises and equipment by classification (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Accumulated depreciation	$ (115,711)	$ (108,864)
Premises and equipment, net	134,088	126,771
Building [Member]
Property Plant And Equipment [Line Items]
Premises and Equipment, gross	$ 145,051	138,901
Building [Member] | Minimum [Member]
Property Plant And Equipment [Line Items]
Estimated Useful Lives	10 years
Building [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	$ 69,915	64,293
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	$ 34,833	$ 32,441