Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Other Intangible Assets [Line Items]
Amortization of Intangible Assets	$ 4,200,000	$ 4,100,000	$ 3,600,000
Goodwill	172,559,000	172,559,000	112,657,000
Additional goodwill	0	59,902,000
Goodwill impairment	0	0	0
Material impairments to identifiable intangible assets	0	0	$ 0
Mortgage Servicing Rights [Member]
Goodwill And Other Intangible Assets [Line Items]
Aggregate fair value of mortgage servicing rights	$ 10,000,000	$ 9,500,000
Range of discount rates used for valuation of mortgage servicing rights	10.00%	10.00%
SBA Servicing Rights [Member]
Goodwill And Other Intangible Assets [Line Items]
Aggregate fair value of mortgage servicing rights	$ 21,000	$ 0