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Marketable Securities and Fair Value Measurements - Short-term and Long-term Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Short-term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 359,956 $ 254,843
Gross Unrealized Gains 14 61
Gross Unrealized Losses (389) (117)
Fair Value 359,581 254,787
Short-term Investments | Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 38,537 33,998
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 38,537 33,998
Short-term Investments | Corporate bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 179,765 152,055
Gross Unrealized Gains 6 27
Gross Unrealized Losses (251) (116)
Fair Value 179,520 151,966
Short-term Investments | U.S. dollar dominated foreign corporate bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 510 901
Gross Unrealized Gains 0 0
Gross Unrealized Losses (2) 0
Fair Value 508 901
Short-term Investments | Municipal Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 14,209 9,147
Gross Unrealized Gains 7 13
Gross Unrealized Losses (2) 0
Fair Value 14,214 9,160
Short-term Investments | U.S. government agency bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 75,172 41,574
Gross Unrealized Gains 0 14
Gross Unrealized Losses (53) (1)
Fair Value 75,119 41,587
Short-term Investments | Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 51,763 1,398
Gross Unrealized Gains 1 0
Gross Unrealized Losses (81) 0
Fair Value 51,683 1,398
Short-term Investments | U.S. Government Treasury Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost   15,770
Gross Unrealized Gains   7
Gross Unrealized Losses   0
Fair Value   15,777
Long Term Investments    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 151,869 148,030
Gross Unrealized Gains 9 28
Gross Unrealized Losses (507) (166)
Fair Value 151,371 147,892
Long Term Investments | Corporate bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 64,012 57,195
Gross Unrealized Gains 9 6
Gross Unrealized Losses (217) (112)
Fair Value 63,804 57,089
Long Term Investments | U.S. dollar dominated foreign corporate bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost   523
Gross Unrealized Gains   0
Gross Unrealized Losses   (2)
Fair Value   521
Long Term Investments | Municipal Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,993 9,552
Gross Unrealized Gains 0 5
Gross Unrealized Losses (2) (6)
Fair Value 3,991 9,551
Long Term Investments | Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 2,338 11,713
Gross Unrealized Gains 0 0
Gross Unrealized Losses (3) (12)
Fair Value 2,335 11,701
Long Term Investments | US Government Agency Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 43,853 48,233
Gross Unrealized Gains 0 12
Gross Unrealized Losses (178) (28)
Fair Value 43,675 48,217
Long Term Investments | U.S. Government Treasury Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 37,673 20,814
Gross Unrealized Gains 0 5
Gross Unrealized Losses (107) (6)
Fair Value $ 37,566 $ 20,813