Fair Value Measurements - Gross Unrealized Loss Positions (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 59,808	$ 76,199
Unrealized Losses, Less than 12 Months	(754)	(448)
Fair Value, More than 12 Months	73,595	71,712
Unrealized Losses, More than 12 Months	(763)	(714)
Fair Value, Total	133,403	147,911
Unrealized Losses, Total	(1,517)	(1,162)
U.S. Government and sponsored entities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	22,889	23,023
Unrealized Losses, Less than 12 Months	(346)	(206)
Fair Value, More than 12 Months	36,008	35,989
Unrealized Losses, More than 12 Months	(427)	(454)
Fair Value, Total	58,897	59,012
Unrealized Losses, Total	(773)	(660)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	32,229	45,857
Unrealized Losses, Less than 12 Months	(349)	(207)
Fair Value, More than 12 Months	33,293	32,634
Unrealized Losses, More than 12 Months	(313)	(243)
Fair Value, Total	65,522	78,491
Unrealized Losses, Total	(662)	(450)
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	377	378
Unrealized Losses, Less than 12 Months	(3)	(2)
Fair Value, More than 12 Months	0	0
Unrealized Losses, More than 12 Months	0	0
Fair Value, Total	377	378
Unrealized Losses, Total	(3)	(2)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	4,302	6,779
Unrealized Losses, Less than 12 Months	(56)	(31)
Fair Value, More than 12 Months	4,134	2,947
Unrealized Losses, More than 12 Months	(21)	(16)
Fair Value, Total	8,436	9,726
Unrealized Losses, Total	(77)	(47)
Mortgage-backed securities – residential
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	11	162
Unrealized Losses, Less than 12 Months	0	(2)
Fair Value, More than 12 Months	160	142
Unrealized Losses, More than 12 Months	(2)	(1)
Fair Value, Total	171	304
Unrealized Losses, Total	$ (2)	$ (3)