Borrowings - Additional Information (Detail)	1 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Mar. 31, 2007 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2007 USD ($) Security
Borrowings Under Repurchase Agreements [Line Items]
Unsecured line of credit facility, maximum borrowing capacity			$ 10,000,000	$ 10,000,000
Line of credit, outstanding amount			0	$ 0
FHLB borrowings, available capacity			532,809,000
FHLB borrowings, pledge of selected securities, loan covered amount			8,068,000
FHLB borrowings, pledge on certain loans, loan covered amount			1,085,644,000
Borrowings from unaffiliated entities, interest rate				5.25%
Issuance of preferred securities | Security						2
Proceeds from subordinated borrowings	$ 50,000,000		$ 0	$ 50,000,000	$ 0
Fixed Interest rate loan	5.75%
London Interbank Offered Rate (LIBOR) [Member]
Borrowings Under Repurchase Agreements [Line Items]
Interest rate	4.55%
Unaffiliated Institution [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings from unaffiliated entities, transaction amount		$ 10,000,000
Borrowings securities pledged under certain transaction				11,416,000
Borrowing maturity date			Mar. 20, 2017
Over Night Borrowings [Member] | Unaffiliated institution 1 [Member]
Borrowings Under Repurchase Agreements [Line Items]
Borrowings under US treasury			$ 0	$ 0
Subordinated Debentures [Member]
Borrowings Under Repurchase Agreements [Line Items]
Issue of floating rate trust preferred securities						$ 10,000,000
Trust preferred securities, interest payment deferment period maximum			5 years
Borrowings, floating interest rate description			Floats based on the 3 month LIBOR plus 1.55% and was 3.14% at December 31, 2017 and 2.51% at December 31, 2016.
Line of credit, basis spread on LIBOR			1.55%
Trust preferred securities, period end interest rate			3.14%	2.51%
Trust preferred securities, redemption year maximum			2037