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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | $ 331,622 | $ 460,252
Less Than 12 Months, Unrealized Losses | (792) | (1,777)
12 Months or Greater, Estimated Fair Value | 245,745 | 35,566
12 Months or Greater, Unrealized Losses | (1,376) | (74)
Total Estimated Fair Value | 577,367 | 495,818
Total Unrealized Losses | (2,168) | (1,851)
Commercial paper |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 51,927 | 62,602
Less Than 12 Months, Unrealized Losses | (45) | (35)
12 Months or Greater, Estimated Fair Value | 0 | 0
12 Months or Greater, Unrealized Losses | 0 | 0
Total Estimated Fair Value | 51,927 | 62,602
Total Unrealized Losses | (45) | (35)
Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 274,696 | 386,728
Less Than 12 Months, Unrealized Losses | (746) | (1,660)
12 Months or Greater, Estimated Fair Value | 234,798 | 28,087
12 Months or Greater, Unrealized Losses | (1,321) | (50)
Total Estimated Fair Value | 509,494 | 414,815
Total Unrealized Losses | (2,067) | (1,710)
Securities of government-sponsored entities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 4,999 | 10,922
Less Than 12 Months, Unrealized Losses | (1) | (82)
12 Months or Greater, Estimated Fair Value | 10,947 | 7,479
12 Months or Greater, Unrealized Losses | (55) | (24)
Total Estimated Fair Value | 15,946 | 18,401
Total Unrealized Losses | $ (56) | $ (106)

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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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