Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 199,736	$ 130,018
Unrealized Gains	3	2
Unrealized Losses	(339)	(282)
Fair Value	199,400	129,738
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	19,280
Unrealized Losses	(14)
Fair Value	19,266
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	735
Fair Value	735
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	179,721	130,018
Unrealized Gains	3	2
Unrealized Losses	(325)	(282)
Fair Value	$ 179,399	$ 129,738

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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