Marketable Securities - Summary of Available -for-sale Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Amortized Cost	$ 257,014	$ 255,231
Gross Unrealized Gains	341	3
Gross Unrealized Losses	(5)	(31)
Estimated Fair Value	257,350	255,203
U.S. Treasury Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	176,443	165,959
Gross Unrealized Gains	188	2
Gross Unrealized Losses	(5)	(13)
Estimated Fair Value	176,626	165,948
Financial Institution Debt Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	53,699	65,436
Gross Unrealized Gains	124	1
Gross Unrealized Losses		(17)
Estimated Fair Value	53,823	65,420
Corporate Debt Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	18,900	23,836
Gross Unrealized Gains	6
Gross Unrealized Losses		(1)
Estimated Fair Value	18,906	$ 23,835
Other Asset Backed Securities [Member]
Marketable Securities [Line Items]
Amortized Cost	7,972
Gross Unrealized Gains	23
Estimated Fair Value	$ 7,995

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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