Income Taxes - Summary of Income Tax Provision Reconciled to Income Taxes that Computed at Statutory Federal Rate (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax, at statutory rates	$ 28,017	$ 21,994	$ 17,028
Increase (deduction) in taxes resulting from:
Tax-exempt income	(1,652)	(1,671)	(1,467)
Excise tax on real estate investment trust (REIT) dividend	1,945	0	0
Adjustment to net deferred tax asset for Tax Cuts and Jobs Act	(1,343)	0	0
State income tax, net of federal income tax effect	931	552	132
Excess tax benefits from employee share-based payments	(587)	0	0
Other, net	158	446	474
Total provision for income taxes	$ 27,469	$ 21,321	$ 16,167