Premises and Equipment - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Building	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Building Terms	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Depreciation	$ 5,900	$ 5,200	$ 11,772	$ 10,406
Number of additional successive terms extendable at the option | Terms			4
Period of extendable lease term, in years			5 years
Lease term, description			Two years and six months
Rent expense	4,500	$ 3,800	$ 8,900	$ 7,700
Sale leaseback transaction, gain recognized over lease term	$ 7,300		$ 7,300		$ 8,200
Number of branch buildings under capital leases | Building	2		2
Minimum [Member]
Property, Plant and Equipment [Line Items]
Lease Term			2 years 6 months
Maximum [Member]
Property, Plant and Equipment [Line Items]
Lease Term			25 years
Prior Sale Leaseback Transactions [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Lease Term			5 years
Prior Sale Leaseback Transactions [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Lease Term			24 years