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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, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Estimated Fair Value	$ 305,930	$ 507,749
Equity securities, Amortized Cost	305,899
Equity securities, Unrealized Gains	74
Equity securities, Unrealized Losses	(43)
Equity securities, Estimated Fair Value	305,930
Debt and equity securities, Amortized Cost	305,899	507,465
Debt and equity securities, Unrealized Gains	74	335
Debt and equity securities, Unrealized Losses	(43)	(51)
Debt and equity securities, Estimated Fair Value	305,930	507,749
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt securities, Amortized Cost	205,111	228,609
Debt securities, Unrealized Gains	6	7
Debt securities, Unrealized Losses	(27)	(13)
Debt securities, Estimated Fair Value	205,090	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	10,004	88,920
Debt securities, Unrealized Gains		5
Debt securities, Unrealized Losses	(5)	(30)
Debt securities, Estimated Fair Value	9,999	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	52,341	87,785
Debt securities, Unrealized Gains	47	262
Debt securities, Unrealized Losses		(8)
Debt securities, Estimated Fair Value	52,388	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	38,443	102,151
Debt securities, Unrealized Gains	21	61
Debt securities, Unrealized Losses	(11)
Debt securities, Estimated Fair Value	$ 38,453	$ 102,212
Equity securities, Amortized Cost			102,151
Equity securities, Unrealized Gains			61
Equity securities, Estimated Fair Value			$ 102,212

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