Income Taxes - Summary of Difference in Effective Tax Rates and Statutory Federal Income Tax Rate of 35% (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax at statutory rate, amount									$ 11,578	$ 9,440	$ 8,461
Tax exempt income, amount									(553)	(471)	(45)
Life insurance, amount									(569)	(523)	(602)
Tax exempt insurance premiums, amount									(303)
Equity compensation, amount									(246)
Enactment of federal tax reform, amount									1,545
Disproportionate tax reversal, amount										(511)
Other, amount									(157)	208	79
Income tax provision (benefit), total amount	$ 4,294	$ 3,067	$ 3,377	$ 557	$ 1,734	$ 2,288	$ 2,529	$ 1,592	$ 11,295	$ 8,143	$ 7,893
Tax at statutory rate, rate									35.00%	35.00%	35.00%
Tax exempt income, rate									(1.70%)	(1.70%)	(0.20%)
Life insurance, rate									(1.70%)	(1.90%)	(2.50%)
Tax exempt insurance premiums, rate									(0.90%)
Equity compensation, rate									(0.70%)
Enactment of federal tax reform, rate									4.60%
Disproportionate tax reversal, rate										(1.90%)
Other, rate									(0.50%)	0.70%	0.30%
Income tax provision, total rate									34.10%	30.20%	32.60%