Loans - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan	Dec. 31, 2016 USD ($)
Accounts, Notes, Loans and Financing Receivable
Non-covered loans net of unearned premiums and deferred loan fees	$ 18,500,000		$ 18,500,000		$ 15,300,000
Outstanding balances for PCI loans	1,600,000,000		1,600,000,000		1,900,000,000
Transfer of loans to loans held for sale			225,541,000	$ 174,501,000
Sales of loans	0	$ 0	0	0
FDIC loss-share receivable	6,800,000		6,800,000		7,200,000
Impaired loans	250,000		$ 250,000
Number of consecutive months for performing restructured loans to be put on accrual status			6 months
TDRs not reported as non-accrual loans	113,700,000		$ 113,700,000		85,200,000
Non-performing TDRs	18,700,000		18,700,000		10,600,000
Specific reserves for loan losses			5,300,000	2,400,000
Partial loan charge-offs related to loans modified as TDRs	0			$ 209,000
Recorded Investment	0
Other real estate owned	10,800,000		10,800,000		10,200,000
Other real estate owned covered by loss sharing agreements	0		0		558,000
Residential real estate properties
Accounts, Notes, Loans and Financing Receivable
Other real estate owned	7,200,000		7,200,000		1,600,000
In formal foreclosure proceedings
Accounts, Notes, Loans and Financing Receivable
Residential mortgage and consumer loans secured by residential real estate properties	4,900,000		$ 4,900,000		7,100,000
Commercial real estate
Accounts, Notes, Loans and Financing Receivable
Number of contracts (in contract) | SecurityLoan			1
Recorded Investment			$ 732,000
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable
Number of contracts (in contract) | SecurityLoan			7
Recorded Investment			$ 6,400,000
Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Transfer of loans to loans held for sale			225,500,000
Number of contracts (in contract) | SecurityLoan				4
Recorded Investment		$ 1,100,000		$ 1,100,000
PCI Loans
Accounts, Notes, Loans and Financing Receivable
Total PCI Loans including covered loans	$ 42,600,000		$ 42,600,000		$ 70,400,000