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Marketable Securities - Schedule of Available-for-Sale Securities Reconciliation (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 1,002,521 | $ 2,949,502
Unrealized Gain | 58 | 1,049
Unrealized Loss | (3,104) | (4,793)
Fair Value | 999,475 | 2,945,758
US Treasury Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 164,619 | 593,599
Unrealized Gain | 16 | 82
Unrealized Loss | (320) | (1,693)
Fair Value | 164,315 | 591,988
Foreign Government Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 3,486 | 6,982
Unrealized Gain | 1 |
Unrealized Loss | (24) | (30)
Fair Value | 3,463 | 6,952
Corporate Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 725,778 | 1,977,329
Unrealized Gain | 41 | 897
Unrealized Loss | (2,760) | (3,066)
Fair Value | 723,059 | 1,975,160
Time Deposits [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 108,638 | 371,515
Unrealized Loss |  | (4)
Fair Value | 108,638 | 371,511
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 77
Unrealized Gain |  | 70
Fair Value |  | 147
Cash Equivalents [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 60,532 | 194,377
Unrealized Gain | 0 |
Unrealized Loss | (1) | (1)
Fair Value | 60,531 | 194,376
Short-term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 941,989 | 2,755,125
Unrealized Gain | 58 | 1,049
Unrealized Loss | (3,103) | (4,792)
Fair Value | $ 938,944 | $ 2,751,382

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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