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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) item | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($)
Available-for-sale securities |  |  |
Number of objectives of the entity's investment strategy | item | 2 |  |
Realized gains | $ 30 | $ 4 |
Other than temporary declines in value | $ 0 |  |
Minimum contractual term of certain current investments which can be liquidated | 1 year |  |
Amortized Cost | $ 895,769 |  | $ 347,913
Gains in Accumulated Other Comprehensive Income (Loss) | 4 |  | 11
Losses in Accumulated Other Comprehensive Income (Loss) | (2,299) |  | (700)
Estimated Fair Value | 893,474 |  | 347,224
Realized losses | 0 | $ 0 |
Corporate bonds |  |  |
Available-for-sale securities |  |  |
Amortized Cost | 346,193 |  | 181,639
Gains in Accumulated Other Comprehensive Income (Loss) | 3 |  | 10
Losses in Accumulated Other Comprehensive Income (Loss) | (1,073) |  | (344)
Estimated Fair Value | 345,123 |  | 181,305
Asset backed securities |  |  |
Available-for-sale securities |  |  |
Amortized Cost | 254,390 |  | 94,700
Losses in Accumulated Other Comprehensive Income (Loss) | (843) |  | (185)
Estimated Fair Value | 253,547 |  | 94,515
U.S. government agency securities |  |  |
Available-for-sale securities |  |  |
Amortized Cost | 194,307 |  | 54,974
Losses in Accumulated Other Comprehensive Income (Loss) | (283) |  | (162)
Estimated Fair Value | 194,024 |  | 54,812
Commercial paper |  |  |
Available-for-sale securities |  |  |
Amortized Cost | 52,015 |  | 9,953
Losses in Accumulated Other Comprehensive Income (Loss) | (21) |  | (7)
Estimated Fair Value | 51,994 |  | 9,946
Certificates of deposit |  |  |
Available-for-sale securities |  |  |
Amortized Cost | 48,864 |  | 6,647
Gains in Accumulated Other Comprehensive Income (Loss) | 1 |  | 1
Losses in Accumulated Other Comprehensive Income (Loss) | (79) |  | (2)
Estimated Fair Value | $ 48,786 |  | $ 6,646

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- Definition Represents the minimum contractual term of certain current investments which the company has the ability and intent, if necessary, to liquidate in order to support its current operations. + References No definition available. + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt 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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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt 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 (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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