Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 49,276	$ 54,131
Gross Unrealized Gain		3,952
Gross Unrealized Loss	(28)	(24)
Fair Value	49,248	58,059
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,603	8,991
Fair Value	6,603	8,991
United States government agency securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,780	8,030
Gross Unrealized Loss	(10)	(1)
Fair Value	14,770	8,029
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,893	37,110
Gross Unrealized Loss	(18)	(23)
Fair Value	$ 27,875	37,087
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Gain		3,952
Fair Value		$ 3,952

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
-Paragraph 1
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