Fair Value Measurements - Summary of Fair Value and Duration of Investments, Including Cash Equivalents, that have been Classified in Gross Unrealized Loss Position (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 76,199
Unrealized Losses, Less than 12 Months	(448)
Fair Value, More than 12 Months	71,712
Unrealized Losses, More than 12 Months	(714)
Fair Value, Total	147,911
Unrealized Losses, Total	(1,162)
U.S. Government and sponsored entities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	23,023
Unrealized Losses, Less than 12 Months	(206)
Fair Value, More than 12 Months	35,989
Unrealized Losses, More than 12 Months	(454)
Fair Value, Total	59,012
Unrealized Losses, Total	(660)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	45,857
Unrealized Losses, Less than 12 Months	(207)
Fair Value, More than 12 Months	32,634
Unrealized Losses, More than 12 Months	(243)
Fair Value, Total	78,491
Unrealized Losses, Total	(450)
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	378
Unrealized Losses, Less than 12 Months	(2)
Fair Value, More than 12 Months	0
Unrealized Losses, More than 12 Months	0
Fair Value, Total	378
Unrealized Losses, Total	(2)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	6,779
Unrealized Losses, Less than 12 Months	(31)
Fair Value, More than 12 Months	2,947
Unrealized Losses, More than 12 Months	(16)
Fair Value, Total	9,726
Unrealized Losses, Total	(47)
Mortgage-backed securities – residential
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	162
Unrealized Losses, Less than 12 Months	(2)
Fair Value, More than 12 Months	142
Unrealized Losses, More than 12 Months	(1)
Fair Value, Total	304
Unrealized Losses, Total	$ (3)