Loans - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan losses	$ 20,446,000	$ 21,610,000	$ 20,443,000	$ 21,202,000
Amounts charged off	$ 433,000	271,000
Troubled debt restructuring period past due considered payment default	30 days
Period of cumulative homogeneous loans past due included in company analysis	90 days
Purchased credit impairment loans, outstanding	$ 2,229,357,000		2,190,662,000
Purchased Credit Impaired Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Increment in allowance for loan losses	0	0
Purchased credit impairment loans, outstanding	0		0
Troubled Debt Restructuring [Member]
Accounts Notes And Loans Receivable [Line Items]
Recorded investment in trouble debt restructuring	16,600,000		16,600,000
Allowance for loan losses	1,100,000		1,100,000
Commitment to lend additional amounts	43,000		$ 41,000
Increment in allowance for loan losses	0	0
Amounts charged off	$ 0	0
TDRs with Subsequent Default [Member]
Accounts Notes And Loans Receivable [Line Items]
Increment in allowance for loan losses		0
Amounts charged off		$ 0