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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 460,252 $ 241,956
Less Than 12 Months, Unrealized Losses (1,777) (321)
12 Months or Greater, Estimated Fair Value 35,566 9,144
12 Months or Greater, Unrealized Losses (74) (3)
Total Estimated Fair Value 495,818 251,100
Total Unrealized Losses (1,851) (324)
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 62,602 43,781
Less Than 12 Months, Unrealized Losses (35) (38)
12 Months or Greater, Estimated Fair Value 0 0
12 Months or Greater, Unrealized Losses 0 0
Total Estimated Fair Value 62,602 43,781
Total Unrealized Losses (35) (38)
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 386,728 185,243
Less Than 12 Months, Unrealized Losses (1,660) (261)
12 Months or Greater, Estimated Fair Value 28,087 9,144
12 Months or Greater, Unrealized Losses (50) (3)
Total Estimated Fair Value 414,815 194,387
Total Unrealized Losses (1,710) (264)
Domestic and foreign government agencies debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 10,922 12,932
Less Than 12 Months, Unrealized Losses (82) (22)
12 Months or Greater, Estimated Fair Value 7,479 0
12 Months or Greater, Unrealized Losses (24) 0
Total Estimated Fair Value 18,401 12,932
Total Unrealized Losses $ (106) $ (22)