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Cash, Cash Equivalents and Investments - Gross Unrealized Losses and Fair Value for Available-for-Sale Investments (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Schedule of Available-for-sale Securities [Line Items]  
Fair Value, Less than 12 months $ 361,492
Unrealized Losses, Less than 12 months (837)
Fair Value, Greater than 12 Months 65,139
Unrealized Losses, Greater than 12 Months (327)
Fair Value 426,631
Unrealized Losses (1,164)
Corporate Bonds [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Fair Value, Less than 12 months 126,659
Unrealized Losses, Less than 12 months (241)
Fair Value, Greater than 12 Months 19,324
Unrealized Losses, Greater than 12 Months (178)
Fair Value 145,983
Unrealized Losses (419)
Treasury Bills [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Fair Value, Less than 12 months 145,051
Unrealized Losses, Less than 12 months (403)
Fair Value, Greater than 12 Months 25,176
Unrealized Losses, Greater than 12 Months (60)
Fair Value 170,227
Unrealized Losses (463)
Asset-Backed Securities [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Fair Value, Less than 12 months 75,842
Unrealized Losses, Less than 12 months (171)
Fair Value, Greater than 12 Months 2,698
Unrealized Losses, Greater than 12 Months (5)
Fair Value 78,540
Unrealized Losses (176)
Sovereign Bonds [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Fair Value, Less than 12 months 3,809
Unrealized Losses, Less than 12 months (4)
Fair Value, Greater than 12 Months 17,941
Unrealized Losses, Greater than 12 Months (84)
Fair Value 21,750
Unrealized Losses (88)
Municipal Bonds [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Fair Value, Less than 12 months 4,203
Unrealized Losses, Less than 12 months (16)
Fair Value, Greater than 12 Months 0
Fair Value 4,203
Unrealized Losses (16)
Agency Bonds [Member]  
Schedule of Available-for-sale Securities [Line Items]  
Fair Value, Less than 12 months 5,928
Unrealized Losses, Less than 12 months (2)
Fair Value, Greater than 12 Months 0
Unrealized Losses, Greater than 12 Months 0
Fair Value 5,928
Unrealized Losses $ (2)