Summary of Cash, Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	$ 250,881	$ 191,934
Unrealized Gains	6
Unrealized Losses	(107)	(80)
Estimated Fair Value	250,780	191,854
Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	36,067	26,584
Estimated Fair Value	36,067	26,584
Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	214,814	165,350
Unrealized Gains	6
Unrealized Losses	(107)	(80)
Estimated Fair Value	214,713	165,270
Cash \| Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	1,908	4,041
Estimated Fair Value	1,908	4,041
Money Market Funds \| Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	25,923	22,543
Estimated Fair Value	25,923	22,543
U.S. Treasuries \| Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	4,000
Estimated Fair Value	4,000
U.S. Treasuries \| Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	45,218	45,559
Unrealized Losses	(12)	(25)
Estimated Fair Value	45,206	45,534
Corporate Debt Securities \| Cash and Cash Equivalents
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	4,236
Estimated Fair Value	4,236
Corporate Debt Securities \| Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	114,931	32,931
Unrealized Gains	5
Unrealized Losses	(73)	(15)
Estimated Fair Value	114,863	32,916
U.S. Government Agency Securities \| Marketable Securities Available-for-Sale
Cash Cash Equivalents And Marketable Securities [Line Items]
Amortized Cost	54,665	86,860
Unrealized Gains	1
Unrealized Losses	(22)	(40)
Estimated Fair Value	$ 54,644	$ 86,820

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

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