Investment Securities - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Carrying value of securities pledged to secure public deposits and other contractual obligations	$ 345,100	$ 356,700
Pledging requirements for credit derivatives and SWAP agreements - securities	$ 1,800	$ 1,400
Number of investments in non-federal issuer representing more than 10% of shareholders' equity | Investment	0	0
Maximum investment in any single non-federal issuer representing shareholders' equity	10.00%	10.00%
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year	129
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 6,714	$ 1,981
Mortgage-Backed Securities [Domain]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year	104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 3,100
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than temporary impairment charges	0	0	$ 5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0
U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year	4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 90	0
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year	14
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 3,459	1,896
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale, securities in unrealized loss positions, qualitative disclosure, number of positions, greater than or equal to one year	7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 32	$ 6