Cash, Cash Equivalents and Investments - Amortized Cost to Fair Value (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 721,460
Gross Unrealized Gains	1,106
Gross Unrealized Losses	(1,164)
Fair Value, Total	721,402
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	343,409
Corporate Bonds [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,391
Gross Unrealized Gains	29
Gross Unrealized Losses	(25)
Fair Value, Total	47,395
Corporate Bonds [Member] \| Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	295,427
Gross Unrealized Gains	981
Gross Unrealized Losses	(394)
Fair Value, Total	296,014
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	25,498
Agency Bonds [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,600
Gross Unrealized Gains	8
Gross Unrealized Losses	0
Fair Value, Total	10,608
Agency Bonds [Member] \| Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,878
Gross Unrealized Gains	14
Gross Unrealized Losses	(2)
Fair Value, Total	14,890
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	130,930
Asset-Backed Securities [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	59,267
Gross Unrealized Gains	8
Gross Unrealized Losses	(72)
Fair Value, Total	59,203
Asset-Backed Securities [Member] \| Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	71,801
Gross Unrealized Gains	30
Gross Unrealized Losses	(104)
Fair Value, Total	71,727
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	13,009
Municipal Bonds [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,805
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value, Total	8,805
Municipal Bonds [Member] \| Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,220
Gross Unrealized Gains	0
Gross Unrealized Losses	(16)
Fair Value, Total	4,204
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	173,830
Treasury Bills [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	150,726
Gross Unrealized Gains	0
Gross Unrealized Losses	(355)
Fair Value, Total	150,371
Treasury Bills [Member] \| Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,567
Gross Unrealized Gains	0
Gross Unrealized Losses	(108)
Fair Value, Total	23,459
Sovereign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Total	34,726
Sovereign Bonds [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,607
Gross Unrealized Gains	1
Gross Unrealized Losses	(29)
Fair Value, Total	21,579
Sovereign Bonds [Member] \| Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,171
Gross Unrealized Gains	35
Gross Unrealized Losses	(59)
Fair Value, Total	$ 13,147

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=73527792&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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