Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 361,370	$ 395,304
Unrealized Gains	33,181	129
Unrealized Losses	(3)	(414)
Fair Value	394,548	395,019
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	102,825	103,410
Unrealized Gains	388	93
Unrealized Losses		(37)
Fair Value	103,213	103,466
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	8,748	8,992
Unrealized Gains	67
Fair Value	8,815	8,992
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	249,377	282,902
Unrealized Gains	1,070	36
Unrealized Losses	(3)	(377)
Fair Value	250,444	$ 282,561
Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	420
Unrealized Gains	31,656
Fair Value	$ 32,076

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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