Investments and Fair Value Measurements - Summary of Fair Value and Duration of Investments, Including Cash Equivalents, that have been Classified in Gross Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 67,160	$ 76,199
Less than 12 Months, Unrealized Losses	(781)	(448)
More than 12 Months, Fair Value	72,910	71,712
More than 12 Months, Unrealized Losses	(901)	(714)
Total, Fair Value	140,070	147,911
Total, Unrealized Losses	(1,682)	(1,162)
U.S. Government and Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	22,912	23,023
Less than 12 Months, Unrealized Losses	(319)	(206)
More than 12 Months, Fair Value	35,948	35,989
More than 12 Months, Unrealized Losses	(491)	(454)
Total, Fair Value	58,860	59,012
Total, Unrealized Losses	(810)	(660)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	37,448	45,857
Less than 12 Months, Unrealized Losses	(394)	(207)
More than 12 Months, Fair Value	34,090	32,634
More than 12 Months, Unrealized Losses	(386)	(243)
Total, Fair Value	71,538	78,491
Total, Unrealized Losses	(780)	(450)
Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	377	378
Less than 12 Months, Unrealized Losses	(3)	(2)
Total, Fair Value	377	378
Total, Unrealized Losses	(3)	(2)
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	6,317	6,779
Less than 12 Months, Unrealized Losses	(64)	(31)
More than 12 Months, Fair Value	2,728	2,947
More than 12 Months, Unrealized Losses	(23)	(16)
Total, Fair Value	9,045	9,726
Total, Unrealized Losses	(87)	(47)
Mortgage-Backed Securities - Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	106	162
Less than 12 Months, Unrealized Losses	(1)	(2)
More than 12 Months, Fair Value	144	142
More than 12 Months, Unrealized Losses	(1)	(1)
Total, Fair Value	250	304
Total, Unrealized Losses	$ (2)	$ (3)