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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value $ 389,258 $ 241,956
Less Than 12 Months, Unrealized Losses (681) (321)
12 Months or Greater, Estimated Fair Value 12,748 9,144
12 Months or Greater, Unrealized Losses (33) (3)
Total Estimated Fair Value 402,006 251,100
Total Unrealized Losses (714) (324)
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 49,185 43,781
Less Than 12 Months, Unrealized Losses (42) (38)
12 Months or Greater, Estimated Fair Value 0 0
12 Months or Greater, Unrealized Losses 0 0
Total Estimated Fair Value 49,185 43,781
Total Unrealized Losses (42) (38)
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 327,596 185,243
Less Than 12 Months, Unrealized Losses (616) (261)
12 Months or Greater, Estimated Fair Value 12,748 9,144
12 Months or Greater, Unrealized Losses (33) (3)
Total Estimated Fair Value 340,344 194,387
Total Unrealized Losses (649) (264)
Domestic and foreign government agencies debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Less Than 12 Months, Estimated Fair Value 12,477 12,932
Less Than 12 Months, Unrealized Losses (23) (22)
12 Months or Greater, Estimated Fair Value 0 0
12 Months or Greater, Unrealized Losses 0 0
Total Estimated Fair Value 12,477 12,932
Total Unrealized Losses $ (23) $ (22)