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Schedule of Available for Sale Marketable Securities (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | $ 259,011 | $ 1,470,729
Available-for-sale marketable securities, Unrealized Gain | 592 | 6,582
Available-for-sale marketable securities, Unrealized (Loss) | (2,967) | (3,406)
Available-for-sale marketable securities, Fair Market Value | 256,636 | 1,473,905
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 191,978 | 1,350,896
U.S. Treasury Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 110,969 | 858,258
Available-for-sale marketable securities, Unrealized Gain | 112 | 72
Available-for-sale marketable securities, Unrealized (Loss) | (1,360) | (2,535)
Available-for-sale marketable securities, Fair Market Value | 109,721 | 855,795
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 75,040 | 850,163
Commercial Paper |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 86,130 | 283,009
Available-for-sale marketable securities, Unrealized Gain | 13 | 18
Available-for-sale marketable securities, Unrealized (Loss) | (26) | (187)
Available-for-sale marketable securities, Fair Market Value | 86,117 | 282,840
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 85,094 | 258,933
Corporate Debt Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 41,133 | 131,179
Available-for-sale marketable securities, Unrealized Gain | 432 | 2,380
Available-for-sale marketable securities, Unrealized (Loss) | (1,545) | (373)
Available-for-sale marketable securities, Fair Market Value | 40,020 | 133,186
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 24,767 | 91,010
U.S. Government Agency Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 9,646 | 10,775
Available-for-sale marketable securities, Unrealized Gain | 1 |
Available-for-sale marketable securities, Unrealized (Loss) | (36) | (49)
Available-for-sale marketable securities, Fair Market Value | 9,611 | 10,726
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 7,077 | 10,727
Certificates of Deposit and Time Deposits |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 7,604 | 167,523
Available-for-sale marketable securities, Unrealized Gain |  | 6
Available-for-sale marketable securities, Unrealized (Loss) |  | (187)
Available-for-sale marketable securities, Fair Market Value | 7,604 | 167,342
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses |  | 138,340
Debt Mutual Funds |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 3,153 |
Available-for-sale marketable securities, Unrealized Gain | 34 |
Available-for-sale marketable securities, Fair Market Value | 3,187 |
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses | 0 |
Non-U.S. Government Securities |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost | 376 | 582
Available-for-sale marketable securities, Unrealized Gain |  | 4
Available-for-sale marketable securities, Fair Market Value | $ 376 | 586
Equity and Debt Mutual Funds |  |
Schedule of Available-for-sale Securities |  |
Available-for-sale marketable securities, Cost |  | 19,403
Available-for-sale marketable securities, Unrealized Gain |  | 4,102
Available-for-sale marketable securities, Unrealized (Loss) |  | (75)
Available-for-sale marketable securities, Fair Market Value |  | 23,430
Available-for-sale marketable securities, Fair Market Value of Investments with Unrealized Losses |  | $ 1,723

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