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Investments - Carrying Value and Amortized Cost of Company's Investments Summarized by Major Security Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Estimated Fair Value	$ 192,089	$ 507,749
Equity securities, Amortized Cost	191,589
Equity securities, Unrealized Gains	500
Equity securities, Estimated Fair Value	192,089
Debt and equity securities, Amortized Cost	191,589	507,465
Debt and equity securities, Unrealized Gains	500	335
Debt and equity securities, Unrealized Losses		(51)
Debt and equity securities, Estimated Fair Value	192,089	507,749
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	74,004	228,609
Debt securities, Unrealized Gains	134	7
Debt securities, Unrealized Losses		(13)
Debt securities, Estimated Fair Value	74,138	228,603
Equity securities, Amortized Cost		228,609
Equity securities, Unrealized Gains		7
Equity securities, Unrealized Losses		(13)
Equity securities, Estimated Fair Value		228,603
US Government Sponsored Enterprises Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	14,660	88,920
Debt securities, Unrealized Gains	6	5
Debt securities, Unrealized Losses		(30)
Debt securities, Estimated Fair Value	14,666	88,895
Equity securities, Amortized Cost		88,920
Equity securities, Unrealized Gains		5
Equity securities, Unrealized Losses		(30)
Equity securities, Estimated Fair Value		88,895
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	80,513	87,785
Debt securities, Unrealized Gains	286	262
Debt securities, Unrealized Losses		(8)
Debt securities, Estimated Fair Value	80,799	88,039
Equity securities, Amortized Cost		87,785
Equity securities, Unrealized Gains		262
Equity securities, Unrealized Losses		(8)
Equity securities, Estimated Fair Value		88,039
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	22,412	102,151
Debt securities, Unrealized Gains	74	61
Debt securities, Estimated Fair Value	$ 22,486	102,212
Equity securities, Amortized Cost		102,151
Equity securities, Unrealized Gains		61
Equity securities, Estimated Fair Value		$ 102,212

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