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Schedule of Available for Sale Marketable Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | $ 469,504 | $ 259,011
Available-for-sale marketable securities, Unrealized Gain | 3,845 | 592
Available-for-sale marketable securities, Unrealized (Loss) | (438) | (2,967)
Available-for-sale marketable securities, Fair Market Value | 472,911 | 256,636
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 38,277 | 191,978
U.S. Treasury Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 189,927 | 110,969
Available-for-sale marketable securities, Unrealized Gain | 668 | 112
Available-for-sale marketable securities, Unrealized (Loss) | (273) | (1,360)
Available-for-sale marketable securities, Fair Market Value | 190,322 | 109,721
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 18,195 | 75,040
Commercial Paper |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 154,903 | 86,130
Available-for-sale marketable securities, Unrealized Gain | 9 | 13
Available-for-sale marketable securities, Unrealized (Loss) | (2) | (26)
Available-for-sale marketable securities, Fair Market Value | 154,910 | 86,117
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 10,322 | 85,094
Corporate Debt Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 91,553 | 41,133
Available-for-sale marketable securities, Unrealized Gain | 3,035 | 432
Available-for-sale marketable securities, Unrealized (Loss) | (157) | (1,545)
Available-for-sale marketable securities, Fair Market Value | 94,431 | 40,020
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 4,581 | 24,767
Certificates of Deposit and Time Deposits |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 19,717 | 7,604
Available-for-sale marketable securities, Unrealized Gain | 6 |
Available-for-sale marketable securities, Fair Market Value | 19,723 | 7,604
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 2,232 |
U.S. Government Agency Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 9,829 | 9,646
Available-for-sale marketable securities, Unrealized Gain | 15 | 1
Available-for-sale marketable securities, Unrealized (Loss) | (6) | (36)
Available-for-sale marketable securities, Fair Market Value | 9,838 | 9,611
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 2,947 | 7,077
Debt Mutual Funds |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 3,187 | 3,153
Available-for-sale marketable securities, Unrealized Gain | 112 | 34
Available-for-sale marketable securities, Fair Market Value | 3,299 | 3,187
Non-U.S. Government Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 388 | 376
Available-for-sale marketable securities, Fair Market Value | $ 388 | $ 376

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