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Marketable Securities - Schedule of Available-for-Sale Securities Reconciliation (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 2,033,001 | $ 2,949,502
Unrealized Gain | 399 | 1,049
Unrealized Loss | (7,119) | (4,793)
Fair Value | 2,026,281 | 2,945,758
US Treasury Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 426,999 | 593,599
Unrealized Gain | 1 | 82
Unrealized Loss | (1,968) | (1,693)
Fair Value | 425,032 | 591,988
Foreign Government Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 6,987 | 6,982
Unrealized Loss | (40) | (30)
Fair Value | 6,947 | 6,952
Corporate Debt Securities [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,297,659 | 1,977,329
Unrealized Gain | 328 | 897
Unrealized Loss | (5,109) | (3,066)
Fair Value | 1,292,878 | 1,975,160
Time Deposits [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 301,279 | 371,515
Unrealized Loss | (2) | (4)
Fair Value | 301,277 | 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 | 190,931 | 194,377
Unrealized Gain | 1 |
Unrealized Loss | (2) | (1)
Fair Value | 190,930 | 194,376
Short-term Investments [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 1,842,070 | 2,755,125
Unrealized Gain | 398 | 1,049
Unrealized Loss | (7,117) | (4,792)
Fair Value | $ 1,835,351 | $ 2,751,382

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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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://www.xbrl.org/2003/role/presentationRef -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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