Bank Premises and Equipment and Leases - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 8,914,000	$ 7,889,000	$ 8,385,000
Amortization expense	122,000	136,000	136,000
Rent expense related to properties owned by related parties	$ 24,740,000	$ 14,661,000	$ 12,528,000