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Investments - Estimated Fair Value and Gross Unrealized Loss Position for AFS Debt Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Fair Value | $ 53,569 | $ 331,622
Less Than 12 Months, Unrealized Losses | (26) | (792)
12 Months or Greater, Fair Value | 116,149 | 245,745
12 Months or Greater, Unrealized Losses | (93) | (1,376)
Total Fair Value | 169,718 | 577,367
Total Unrealized Losses | (119) | (2,168)
Commercial paper |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Fair Value | 9,984 | 51,927
Less Than 12 Months, Unrealized Losses | (1) | (45)
Total Fair Value | 9,984 | 51,927
Total Unrealized Losses | (1) | (45)
Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Fair Value | 43,585 | 274,696
Less Than 12 Months, Unrealized Losses | (25) | (746)
12 Months or Greater, Fair Value | 111,155 | 234,798
12 Months or Greater, Unrealized Losses | (87) | (1,321)
Total Fair Value | 154,740 | 509,494
Total Unrealized Losses | (112) | (2,067)
Securities of government-sponsored entities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Fair Value |  | 4,999
Less Than 12 Months, Unrealized Losses |  | (1)
12 Months or Greater, Fair Value | 4,994 | 10,947
12 Months or Greater, Unrealized Losses | (6) | (55)
Total Fair Value | 4,994 | 15,946
Total Unrealized Losses | $ (6) | $ (56)

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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- Definition Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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