Securities - Securities Held to Maturity in Unrecognized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Held To Maturity Securities [Line Items]
Fair value, Less than 12 months		$ 4,253
Unrealized loss, Less than 12 months		(48)
Fair value, 12 months or more	$ 62,727	65,640
Unrealized loss, 12 months or more	(1,963)	(3,272)
Total fair value	62,727	69,893
Total unrealized loss	(1,963)	(3,320)
Mortgage-Backed Securities (MBS) [Member]
Schedule Of Held To Maturity Securities [Line Items]
Fair value, 12 months or more	61,605	62,115
Unrealized loss, 12 months or more	(1,958)	(3,203)
Total fair value	61,605	62,115
Total unrealized loss	(1,958)	(3,203)
States and Municipalities [Member]
Schedule Of Held To Maturity Securities [Line Items]
Fair value, Less than 12 months		4,253
Unrealized loss, Less than 12 months		(48)
Fair value, 12 months or more	1,122	3,525
Unrealized loss, 12 months or more	(5)	(69)
Total fair value	1,122	7,778
Total unrealized loss	$ (5)	$ (117)