Cash Equivalents and Short-Term Investments (Detail) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cost	$ 470,050,000	$ 282,683,000
Gains	117,000	8,000
Losses		(5,000)
Fair Value	470,167,000	282,686,000
Other-than-temporary impairments on available-for-sale securities	0	0
Cash equivalents
Debt Securities, Available-for-sale [Line Items]
Fair Value	339,635,000	163,731,000
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Fair Value	130,532,000	118,955,000
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cost	339,635,000	136,258,000
Gains	0	0
Losses		0
Fair Value	339,635,000	136,258,000
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost	61,295,000	77,082,000
Gains	0	0
Losses		0
Fair Value	25,243,000	77,082,000
U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Cost	43,937,000	5,993,000
Gains	57,000	2,000
Losses		(5,000)
Fair Value	43,994,000	5,990,000
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Cost	25,183,000	48,349,000
Gains	60,000	6,000
Losses		0
Fair Value	61,295,000	48,355,000
Overnight repurchase agreements
Debt Securities, Available-for-sale [Line Items]
Cost	0	15,001,000
Gains	0	0
Losses		0
Fair Value	$ 0	$ 15,001,000

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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