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Balance Sheet Components - Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 53,947	$ 71,360
Gross Unrealized Holding Gains	1,412	1,252
Gross Unrealized Holding Losses	(92)	(46)
Fair Value	55,267	72,566
Corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,529	53,985
Gross Unrealized Holding Gains	0	4
Gross Unrealized Holding Losses	(92)	(46)
Fair Value	36,437	53,943
Bond and mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,293	17,249
Gross Unrealized Holding Gains	1,338	1,153
Gross Unrealized Holding Losses	0	0
Fair Value	18,631	18,402
Equity investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	125	126
Gross Unrealized Holding Gains	74	95
Gross Unrealized Holding Losses	0	0
Fair Value	$ 199	$ 221

X

- Definition

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

+ References

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
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-Section 50
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-Publisher FASB
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-Section 45
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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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 2
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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
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+ References

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