Fair Value Measurements - Fair Values and Unrealized Losses on Investments (Detail) - Corporate Debt [Member] - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Less than Twelve Months, Estimated fair value	$ 21,666	$ 80,282
Less than Twelve Months, Gross unrealized losses	(19)	(256)
Twelve Months or More, Estimated fair value	16,916	87,028
Twelve Months or More, Gross unrealized losses	(14)	(782)
Estimated fair value, Total	38,582	167,310
Gross unrealized losses, Total	$ (33)	$ (1,038)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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