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Summary of Investments Classified as Available-For-Sale Securities (Detail) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Aggregate Estimated Fair Value		$ 209,179	$ 267,573
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Aggregate Estimated Fair Value		240	960
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Aggregate Estimated Fair Value		44,392	49,245
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Estimated Fair Value		136,587	204,436
Domestic and foreign government agencies debt securities
Schedule of Available-for-sale Securities [Line Items]
Aggregate Estimated Fair Value		27,960	12,932
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		171,618	224,236
Gross Unrealized Gains	[1]	9	6
Gross Unrealized Losses	[1]	(134)	(159)
Aggregate Estimated Fair Value		171,493	224,083
Short-term investments | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		240	960
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	0	0
Aggregate Estimated Fair Value		$ 240	$ 960
Short-term investments | Certificates of deposit | Maximum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Short-term investments | Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 44,406	$ 49,280
Gross Unrealized Gains	[1]	2	3
Gross Unrealized Losses	[1]	(16)	(38)
Aggregate Estimated Fair Value		$ 44,392	$ 49,245
Short-term investments | Commercial paper | Maximum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Short-term investments | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 118,969	$ 168,548
Gross Unrealized Gains	[1]	7	3
Gross Unrealized Losses	[1]	(112)	(117)
Aggregate Estimated Fair Value		$ 118,864	$ 168,434
Short-term investments | Corporate debt securities | Maximum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Short-term investments | Domestic and foreign government agencies debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 8,003	$ 5,448
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(6)	(4)
Aggregate Estimated Fair Value		$ 7,997	$ 5,444
Short-term investments | Domestic and foreign government agencies debt securities | Maximum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Long-term investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 37,796	$ 43,655
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(110)	(165)
Aggregate Estimated Fair Value		37,686	43,490
Long-term investments | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		17,793	36,149
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(70)	(147)
Aggregate Estimated Fair Value		$ 17,723	$ 36,002
Long-term investments | Corporate debt securities | Minimum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Long-term investments | Corporate debt securities | Maximum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		2 years	2 years
Long-term investments | Domestic and foreign government agencies debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 20,003	$ 7,506
Gross Unrealized Gains	[1]	0	0
Gross Unrealized Losses	[1]	(40)	(18)
Aggregate Estimated Fair Value		$ 19,963	$ 7,488
Long-term investments | Domestic and foreign government agencies debt securities | Minimum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		1 year	1 year
Long-term investments | Domestic and foreign government agencies debt securities | Maximum
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities contractual maturity		2 years	2 years
[1] (1) Unrealized gains and losses are included in other comprehensive loss.

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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 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
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