Securities - Available for Sale and Held to Maturity Securities with Gross Unrealized Losses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	$ 129,513	$ 184,402	$ 75,072
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	984	4,843	348
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	108,127	2,051	13,943
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	3,244	3	400
Available for Sale and Held to Maturity Securities, Total, Fair Value	237,640	186,453	89,015
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	4,228	4,846	748
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	49,586	97,634	39,889
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	364	2,091	111
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	73,058
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	1,812
Available for Sale and Held to Maturity Securities, Total, Fair Value	122,644	97,634	39,889
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	2,176	2,091	111
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	8,145	24,478	14,894
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	37	521	87
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	14,567		12,581
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	423		392
Available for Sale and Held to Maturity Securities, Total, Fair Value	22,712	24,478	27,475
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	460	521	479
Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	60,230	37,663	16,557
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	415	1,305	131
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	13,492
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	627
Available for Sale and Held to Maturity Securities, Total, Fair Value	73,722	37,663	16,557
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	1,042	1,305	131
States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value	11,552	24,627	2,225
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses	168	926	8
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value	7,010	589	1,362
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses	382	1	8
Available for Sale and Held to Maturity Securities, Total, Fair Value	18,562	25,216	3,587
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses	$ 550	927	16
Corporate Bond [Member]
Schedule of Available-for-sale Securities [Line Items]
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Fair Value			1,507
Available for Sale and Held to Maturity Securities, Less Than Twelve Months, Gross Unrealized Losses			11
Available for Sale and Held to Maturity Securities, Over Twelve Months, Fair Value		1,462
Available for Sale and Held to Maturity Securities, Over Twelve Months, Gross Unrealized Losses		2
Available for Sale and Held to Maturity Securities, Total, Fair Value		1,462	1,507
Available for Sale and Held to Maturity Securities, Total, Gross Unrealized Losses		$ 2	$ 11