Investments - Investments in Continuous Unrealized Loss Position for Less Than Twelve Months (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 0	$ 49,541
Unrealized Losses	0	(299)
US Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	5,472
Unrealized Losses	0	(28)
Corporate Bonds and Notes [Member]
Debt Securities, Available-for-sale [Line Items]
Fair Value	0	44,069
Unrealized Losses	$ 0	$ (271)

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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- Definition

Amount of accumulated unrealized loss on 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.

+ References

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- Definition

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+ References

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