Premises and Equipment - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Building	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Building Term	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Depreciation	$ 5,900	$ 5,800	$ 17,666	$ 16,178
Lease term, description			two years and six months
Number of additional successive terms extendable at the option | Term			4
Period of extendable lease term, in years			5 years
Rent expense	4,400	$ 3,900	$ 13,400	$ 11,600
Sale leaseback transaction, gain recognized over lease term	$ 6,900		$ 6,900		$ 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
Minimum [Member] | Prior Sale Leaseback Transactions [Member]
Property, Plant and Equipment [Line Items]
Lease Term			5 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Lease Term			25 years
Maximum [Member] | Prior Sale Leaseback Transactions [Member]
Property, Plant and Equipment [Line Items]
Lease Term			24 years