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Cash, Cash Equivalents, and Investments - Gross Unrealized Losses and Fair Values for Available-for-Sale Investments (Detail)
$ in Thousands
Sep. 30, 2018
USD ($)
Schedule of Available-for-sale Securities [Line Items]  
Fair value, less than 12 months $ 317,195
Unrealized losses, less than 12 months (779)
Fair value, 12 months or greater 149,642
Unrealized losses, 12 months or greater (681)
Total fair value 466,837
Total unrealized losses (1,460)
Treasury Bills  
Schedule of Available-for-sale Securities [Line Items]  
Fair value, less than 12 months 137,108
Unrealized losses, less than 12 months (174)
Fair value, 12 months or greater 63,030
Unrealized losses, 12 months or greater (124)
Total fair value 200,138
Total unrealized losses (298)
Corporate Bonds  
Schedule of Available-for-sale Securities [Line Items]  
Fair value, less than 12 months 116,028
Unrealized losses, less than 12 months (386)
Fair value, 12 months or greater 42,922
Unrealized losses, 12 months or greater (297)
Total fair value 158,950
Total unrealized losses (683)
Asset-Backed Securities  
Schedule of Available-for-sale Securities [Line Items]  
Fair value, less than 12 months 57,678
Unrealized losses, less than 12 months (207)
Fair value, 12 months or greater 38,256
Unrealized losses, 12 months or greater (200)
Total fair value 95,934
Total unrealized losses (407)
Sovereign Bonds  
Schedule of Available-for-sale Securities [Line Items]  
Fair value, less than 12 months 3,527
Unrealized losses, less than 12 months (6)
Fair value, 12 months or greater 4,592
Unrealized losses, 12 months or greater (47)
Total fair value 8,119
Total unrealized losses (53)
Municipal Bonds  
Schedule of Available-for-sale Securities [Line Items]  
Fair value, less than 12 months 2,854
Unrealized losses, less than 12 months (6)
Fair value, 12 months or greater 842
Unrealized losses, 12 months or greater (13)
Total fair value 3,696
Total unrealized losses $ (19)