Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 306,736	$ 395,304
Unrealized Gains	30,821	129
Unrealized Losses	(24)	(414)
Fair Value	337,533	395,019
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	78,093	103,410
Unrealized Gains	296	93
Unrealized Losses	(3)	(37)
Fair Value	78,386	103,466
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	7,529	8,992
Unrealized Gains	87
Fair Value	7,616	8,992
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	220,694	282,902
Unrealized Gains	1,018	36
Unrealized Losses	(21)	(377)
Fair Value	221,691	$ 282,561
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	420
Unrealized Gains	29,420
Fair Value	$ 29,840

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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