Fair Value of Financial Instruments - Summary of Contractual Maturities of Short and Long Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Amortized Cost Basis, Due within one year	$ 722,251	$ 481,071
Amortized Cost Basis, Due after 1 year through 2 years	59,555	11,466
Amortized Cost	781,806	492,537
Aggregate Fair Value, Due within one year	722,767	480,761
Aggregate Fair Value, Due after 1 year through 2 years	59,782	11,450
Aggregate Fair Value, Total	$ 782,549	$ 492,211

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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