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Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 98,076	$ 50,868
Gross Unrealized Loss	(417)	(97)
Fair Value	97,659	50,771
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	54	3,758
Fair Value	54	3,758
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,940	11,252
Gross Unrealized Loss	(42)	(24)
Fair Value	15,898	11,228
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	82,082	35,858
Gross Unrealized Loss	(375)	(73)
Fair Value	$ 81,707	$ 35,785

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- Definition

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

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