Securities - Summary of Investments in an Unrealized Loss Position that were Temporarily Impaired (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities available for sale:
Less than 12 months - Fair value	$ 34,432	$ 66,073
Less than 12 months - Unrealized (loss)	(339)	(1,429)
12 months or more - Fair value	32,323	5,499
12 months or more - Unrealized (loss)	(1,153)	(208)
Fair Value	66,755	71,572
Unrealized (Loss)	(1,492)	(1,637)
Securities held to maturity:
Less than 12 months - Fair value	25,190	48,980
Less than 12 months - Unrealized (loss)	(250)	(708)
12 months or more - Fair value	13,297	0
12 months or more - Unrealized (loss)	(346)	0
Fair Value	38,487	48,980
Unrealized (Loss)	(596)	(708)
Total securities, Less than 12 months - Fair value	59,622	115,053
Total securities, Less than 12 months - Unrealized (loss)	(589)	(2,137)
Total securities, 12 months or more - Fair value	45,620	5,499
Total securities, 12 months or more - Unrealized (loss)	(1,499)	(208)
Total securities, Fair value	105,242	120,552
Total securities, Unrealized (loss)	(2,088)	(2,345)
U.S. agency and mortgage-backed securities
Securities available for sale:
Less than 12 months - Fair value	29,963	60,943
Less than 12 months - Unrealized (loss)	(286)	(1,249)
12 months or more - Fair value	30,362	5,499
12 months or more - Unrealized (loss)	(1,051)	(208)
Fair Value	60,325	66,442
Unrealized (Loss)	(1,337)	(1,457)
Securities held to maturity:
Less than 12 months - Fair value	18,301	34,770
Less than 12 months - Unrealized (loss)	(205)	(483)
12 months or more - Fair value	13,297	0
12 months or more - Unrealized (loss)	(346)	0
Fair Value	31,598	34,770
Unrealized (Loss)	(551)	(483)
Obligations of states and political subdivisions
Securities available for sale:
Less than 12 months - Fair value	4,469	5,130
Less than 12 months - Unrealized (loss)	(53)	(180)
12 months or more - Fair value	1,961	0
12 months or more - Unrealized (loss)	(102)	0
Fair Value	6,430	5,130
Unrealized (Loss)	(155)	(180)
Securities held to maturity:
Less than 12 months - Fair value	6,889	12,724
Less than 12 months - Unrealized (loss)	(45)	(211)
12 months or more - Fair value	0	0
12 months or more - Unrealized (loss)	0	0
Fair Value	6,889	12,724
Unrealized (Loss)	$ (45)	(211)
Corporate debt securities
Securities held to maturity:
Less than 12 months - Fair value		1,486
Less than 12 months - Unrealized (loss)		(14)
12 months or more - Fair value		0
12 months or more - Unrealized (loss)		0
Fair Value		1,486
Unrealized (Loss)		$ (14)