Summary of Significant Accounting Policies - Summary of Marketable Securities by Security Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 23,508	$ 74,348
Gross Unrealized Gains	5
Gross Unrealized Losses		(68)
Estimated Fair Value	23,513	74,280
United States Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,508	44,587
Gross Unrealized Gains	5
Gross Unrealized Losses		(53)
Estimated Fair Value	$ 23,513	44,534
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		29,761
Gross Unrealized Losses		(15)
Estimated Fair Value		$ 29,746

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-Name Accounting Standards Codification
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