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Gross Unrealized Losses and Fair Value Available-For-Sale Debt securities 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 | $ 234,124 | $ 331,622
Less Than 12 Months, Unrealized Losses | (127) | (792)
12 Months or Greater, Estimated Fair Value | 0 | 245,745
12 Months or Greater, Unrealized Losses | 0 | (1,376)
Total Estimated Fair Value | 234,124 | 577,367
Total Unrealized Losses | (127) | (2,168)
Commercial paper |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 33,070 | 51,927
Less Than 12 Months, Unrealized Losses | (15) | (45)
12 Months or Greater, Estimated Fair Value | 0 | 0
12 Months or Greater, Unrealized Losses | 0 | 0
Total Estimated Fair Value | 33,070 | 51,927
Total Unrealized Losses | (15) | (45)
Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 186,052 | 274,696
Less Than 12 Months, Unrealized Losses | (110) | (746)
12 Months or Greater, Estimated Fair Value | 0 | 234,798
12 Months or Greater, Unrealized Losses | 0 | (1,321)
Total Estimated Fair Value | 186,052 | 509,494
Total Unrealized Losses | (110) | (2,067)
Securities of government-sponsored entities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 15,002 | 4,999
Less Than 12 Months, Unrealized Losses | (2) | (1)
12 Months or Greater, Estimated Fair Value | 0 | 10,947
12 Months or Greater, Unrealized Losses | 0 | (55)
Total Estimated Fair Value | 15,002 | 15,946
Total Unrealized Losses | $ (2) | $ (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, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455 Reference 2: 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=118939692&loc=d3e27337-111563 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=118942371&loc=SL82922954-210456 + 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, without allowance for credit loss. Includes beneficial interest in securitized financial asset. + 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=118939692&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455 + Details
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