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Marketable Securities - Schedule of Available-for-Sale Securities Reconciliation (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 1,682,990 | $ 2,949,502
Unrealized Gain | 457 | 1,049
Unrealized Loss | (5,724) | (4,793)
Fair Value | 1,677,723 | 2,945,758
US Treasury Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 239,150 | 593,599
Unrealized Gain | 3 | 82
Unrealized Loss | (1,394) | (1,693)
Fair Value | 237,759 | 591,988
Foreign Government Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 2,992 | 6,982
Unrealized Loss | (38) | (30)
Fair Value | 2,954 | 6,952
Corporate Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,202,456 | 1,977,329
Unrealized Gain | 384 | 897
Unrealized Loss | (4,291) | (3,066)
Fair Value | 1,198,549 | 1,975,160
Time Deposits [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 238,315 | 371,515
Unrealized Loss | (1) | (4)
Fair Value | 238,314 | 371,511
Equity Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 77 | 77
Unrealized Gain | 70 | 70
Fair Value | 147 | 147
Cash Equivalents [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 171,433 | 194,377
Unrealized Gain | 1 |
Unrealized Loss |  | (1)
Fair Value | 171,434 | 194,376
Short-term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,511,557 | 2,755,125
Unrealized Gain | 456 | 1,049
Unrealized Loss | (5,724) | (4,792)
Fair Value | $ 1,506,289 | $ 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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