Loans - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable
Non-covered loans net of unearned premiums and deferred loan fees	$ 22,000,000		$ 22,200,000
Outstanding balances for PCI loans	5,100,000,000		1,500,000,000
Transfer of loans to loans held for sale	0	$ 103,884,000
Sales of loans	0	0
Impaired loans	$ 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	$ 116,400,000		117,200,000
Non-performing TDRs	24,300,000		$ 27,000,000
Specific reserves for loan losses	958,000	2,000,000
Partial loan charge-offs related to loans modified as TDRs	$ 0	0
Days past due to place on payment default	90 days		90 days
Recorded Investment	$ 165,000	3,128,000
Other real estate owned	13,800,000		$ 9,800,000
Residential real estate properties
Accounts, Notes, Loans and Financing Receivable
Other real estate owned	10,100,000		7,300,000
In formal foreclosure proceedings
Accounts, Notes, Loans and Financing Receivable
Residential mortgage and consumer loans secured by residential real estate properties	3,900,000		3,800,000
Residential mortgage
Accounts, Notes, Loans and Financing Receivable
Transfer of loans to loans held for sale	0	103,900,000
Recorded Investment	0	$ 321,000
PCI Loans
Accounts, Notes, Loans and Financing Receivable
Total PCI Loans including covered loans	$ 33,200,000		$ 38,700,000