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Investments - Carrying Value and Amortized Cost of Company's Investments Summarized by Major Security Type (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Estimated Fair Value	$ 507,749	$ 338,762
Debt and equity securities, Amortized Cost	507,465	339,145
Debt and equity securities, Unrealized Gains	335	39
Debt and equity securities, Unrealized Losses	(51)	(422)
Debt and equity securities, Estimated Fair Value	507,749	338,762
U.S. Treasury notes
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	228,609
Debt securities, Unrealized Gains	7
Debt securities, Unrealized Losses	(13)
Debt securities, Estimated Fair Value	228,603
Government sponsored enterprise securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	88,920
Debt securities, Unrealized Gains	5
Debt securities, Unrealized Losses	(30)
Debt securities, Estimated Fair Value	88,895
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	87,785	187,371
Debt securities, Unrealized Gains	262	39
Debt securities, Unrealized Losses	(8)	(344)
Debt securities, Estimated Fair Value	88,039	187,066
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	102,151	151,774
Debt securities, Unrealized Gains	61
Debt securities, Unrealized Losses		(78)
Debt securities, Estimated Fair Value	$ 102,212	$ 151,696

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