Acquisition - Additional Information (Detail) - USD ($)			3 Months Ended
	Jan. 01, 2018	Oct. 12, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Business Acquisition [Line Items]
Fair value of purchased credit impaired loans			$ 22,752,000	$ 17,670,000	$ 28,080,000
Contractual amounts			28,979,000	23,851,000	42,306,000
Purchased Credit Impaired Loans [Member]
Business Acquisition [Line Items]
Contractual amounts			4,129,000	$ 755,000	$ 1,414,000
FBC Bancshares, Inc. and First Bank, N.A. [Member]
Business Acquisition [Line Items]
Payment for all outstanding shares of acquired entity by shares		1,289,371
Fair value of total loans			266,327,000
Total loans of contractual amounts			271,714,000
FBC Bancshares, Inc. and First Bank, N.A. [Member] | Purchased Credit Impaired Loans [Member]
Business Acquisition [Line Items]
Fair value of purchased credit impaired loans			3,013,000
Contractual amounts			$ 3,806,000
Commercial Bancshares, Inc [Member]
Business Acquisition [Line Items]
Business acquisition, agreement date			Oct. 12, 2017
Payment for all outstanding shares of acquired entity by shares	1,289,371
Business acquisition, special dividend	$ 22,075,000
Increase decrease in business acquisition, special dividend	$ 42,402,000