Summary of Cash, Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	$ 191,934	$ 81,411
Unrealized Gains		8
Unrealized Losses	(80)	(4)
Estimated Fair Value	191,854	81,415
Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	26,584	24,289
Estimated Fair Value	26,584	24,289
Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	165,350	57,122
Unrealized Gains		8
Unrealized Losses	(80)	(4)
Estimated Fair Value	165,270	57,126
Cash \| Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	4,041	3,557
Estimated Fair Value	4,041	3,557
Money Market Funds \| Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	22,543	18,981
Estimated Fair Value	22,543	18,981
U.S. Treasuries \| Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	45,559	3,499
Unrealized Losses	(25)
Estimated Fair Value	45,534	3,499
U.S. Government Agency Securities \| Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost		1,751
Estimated Fair Value		1,751
U.S. Government Agency Securities \| Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	86,860	28,685
Unrealized Gains		3
Unrealized Losses	(40)	(2)
Estimated Fair Value	86,820	28,686
Corporate Debt Securities \| Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	32,931	24,938
Unrealized Gains		5
Unrealized Losses	(15)	(2)
Estimated Fair Value	$ 32,916	$ 24,941

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

+ References

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

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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.

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