Amortized Cost and Fair Value of Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 110,701	$ 109,528
Gross Unrealized Gains	375	155
Gross Unrealized Losses	97	264
Fair Value	110,979	109,419
Money market investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,823	12,395
Fair Value	16,823	12,395
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,497	4,027
Gross Unrealized Gains	2	3
Gross Unrealized Losses	5	19
Fair Value	3,494	4,011
Corporate bonds and direct obligations of government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,885	70,069
Gross Unrealized Gains	373	150
Gross Unrealized Losses	91	239
Fair Value	85,167	69,980
U.S. Treasury obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,496	23,037
Gross Unrealized Gains		2
Gross Unrealized Losses	1	6
Fair Value	$ 5,495	$ 23,033

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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
-Section 25
-Paragraph 1
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-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
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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