Financial Instruments with Off-Balance-Sheet Risk and Concentrations of Credit Risk - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Financial Instruments With Off Balance Sheet Credit Risk [Line Items]
Commitments to lend additional funds to borrowers	$ 20,000	$ 10,000
Standby letters of credit	$ 14,800,000
Expiry of letters of credit	2024
Loan and lease commitments	$ 966,400,000
Loan and lease commitments maturing within one year	620,400,000
Maturing after one year but within three years	181,100,000
Maturing after three years but within five years	42,200,000
Maturing after five years	122,700,000
Letters of credit maturing after one year but within three years	1,900,000
Standby letters of credit maturing within one year	12,900,000
Letters of credit maturing after three year but within five years	31,000
Letters of credit maturing after five years	80,000
Maximum
Schedule Of Financial Instruments With Off Balance Sheet Credit Risk [Line Items]
Standby letters of credit	$ 14,800,000