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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 187,847 $ 241,956
Less Than 12 Months, Unrealized Losses (244) (321)
12 Months or Greater, Estimated Fair Value 0 9,144
12 Months or Greater, Unrealized Losses 0 (3)
Total Estimated Fair Value 187,847 251,100
Total Unrealized Losses (244) (324)
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 38,914 43,781
Less Than 12 Months, Unrealized Losses (16) (38)
12 Months or Greater, Estimated Fair Value 0 0
12 Months or Greater, Unrealized Losses 0 0
Total Estimated Fair Value 38,914 43,781
Total Unrealized Losses (16) (38)
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 120,973 185,243
Less Than 12 Months, Unrealized Losses (182) (261)
12 Months or Greater, Estimated Fair Value 0 9,144
12 Months or Greater, Unrealized Losses 0 (3)
Total Estimated Fair Value 120,973 194,387
Total Unrealized Losses (182) (264)
Domestic and foreign government agencies debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 27,960 12,932
Less Than 12 Months, Unrealized Losses (46) (22)
12 Months or Greater, Estimated Fair Value 0 0
12 Months or Greater, Unrealized Losses 0 0
Total Estimated Fair Value 27,960 12,932
Total Unrealized Losses $ (46) $ (22)