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Marketable Securities - Summary of Cash Equivalents and Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents, at carrying value, total	$ 80,428	$ 140,418
Available-for-sale securities, amortized cost	303,546	259,974
Available-for-sale securities, gross unrealized gains	351	0
Available-for-sale securities, gross unrealized (Losses)	(19)	(259)
Available-for-sale securities, estimated fair value	303,878	259,715
Total cash equivalents and available-for-sale securities, amortized cost	337,040	363,265
Total cash equivalents and available-for-sale securities, gross unrealized gains	352	0
Total cash equivalents and available-for-sale securities, gross unrealized (losses)	(19)	(259)
Total cash equivalents and available-for-sale securities, estimated fair value	337,373	363,006
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents, at carrying value, total	30,496	103,291
Cash equivalents, gross unrealized gains	0	0
Cash and cash equivalents accumulated gross unrealized loss before tax	0	0
Cash equivalents, estimated fair value	30,496	103,291
Commercial paper securities
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents, at carrying value, total	2,998
Cash equivalents, gross unrealized gains	1
Cash and cash equivalents accumulated gross unrealized loss before tax	0
Cash equivalents, estimated fair value	2,999
Available-for-sale securities, amortized cost	155,230	177,353
Available-for-sale securities, gross unrealized gains	145	0
Available-for-sale securities, gross unrealized (Losses)	(7)	(129)
Available-for-sale securities, estimated fair value	155,368	177,224
Cash equivalents:
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents, at carrying value, total	33,494	103,291
Cash equivalents, gross unrealized gains	1	0
Cash and cash equivalents accumulated gross unrealized loss before tax	0	0
Cash equivalents, estimated fair value	33,495	103,291
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	94,905	63,981
Available-for-sale securities, gross unrealized gains	115	0
Available-for-sale securities, gross unrealized (Losses)	(3)	(111)
Available-for-sale securities, estimated fair value	95,017	63,870
U.S. government-sponsored entity debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, amortized cost	53,411	18,640
Available-for-sale securities, gross unrealized gains	91	0
Available-for-sale securities, gross unrealized (Losses)	(9)	(19)
Available-for-sale securities, estimated fair value	$ 53,493	$ 18,621

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