Securities - Securities Held to Maturity in Unrecognized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Held To Maturity Securities [Line Items]
Fair value, Less than 12 months	$ 8,349	$ 608
Unrealized loss, Less than 12 months	(158)	(7)
Fair value, 12 months or more	66,110	72,792
Unrealized loss, 12 months or more	(3,924)	(1,925)
Total fair value	74,459	73,400
Total unrealized loss	(4,082)	(1,932)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule Of Held To Maturity Securities [Line Items]
Fair value, 12 months or more	66,110	72,792
Unrealized loss, 12 months or more	(3,924)	(1,925)
Total fair value	66,110	72,792
Total unrealized loss	(3,924)	(1,925)
States of the U.S. and Political Subdivisions [Member]
Schedule Of Held To Maturity Securities [Line Items]
Fair value, Less than 12 months	8,349	608
Unrealized loss, Less than 12 months	(158)	(7)
Total fair value	8,349	608
Total unrealized loss	$ (158)	$ (7)