Marketable Securities and Fair Value Measurements - Short-term and Long-term Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 98,586	$ 272,280
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(127)	(252)
Fair Value	98,460	272,031
Short-term Investments \| Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,793	58,503
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(1)
Fair Value	17,793	58,502
Short-term Investments \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	45,100	145,728
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(48)	(174)
Fair Value	45,052	145,557
Short-term Investments \| U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	19,981	3,013
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(77)	(7)
Fair Value	19,904	3,006
Short-term Investments \| U.S. Government Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,292	60,650
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	(70)
Fair Value	15,291	60,580
Short-term Investments \| Certificates of Deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	420	4,386
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(1)	0
Fair Value	420	4,386
Long Term Investments
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,091	40,090
Gross Unrealized Gains	23	2
Gross Unrealized Losses	(2)	(144)
Fair Value	9,112	39,948
Long Term Investments \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,957	25,067
Gross Unrealized Gains	5	2
Gross Unrealized Losses	(2)	(76)
Fair Value	4,960	24,993
Long Term Investments \| U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,399	15,023
Gross Unrealized Gains	8	0
Gross Unrealized Losses	0	(68)
Fair Value	1,407	$ 14,955
Long Term Investments \| Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	500
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Fair Value	501
Long Term Investments \| U.S. Government Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,235
Gross Unrealized Gains	9
Gross Unrealized Losses	0
Fair Value	$ 2,244

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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+ References

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