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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | $ 460,252 | $ 241,956
Less Than 12 Months, Unrealized Losses | (1,777) | (321)
12 Months or Greater, Estimated Fair Value | 35,566 | 9,144
12 Months or Greater, Unrealized Losses | (74) | (3)
Total Estimated Fair Value | 495,818 | 251,100
Total Unrealized Losses | (1,851) | (324)
Commercial paper |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 62,602 | 43,781
Less Than 12 Months, Unrealized Losses | (35) | (38)
12 Months or Greater, Estimated Fair Value | 0 | 0
12 Months or Greater, Unrealized Losses | 0 | 0
Total Estimated Fair Value | 62,602 | 43,781
Total Unrealized Losses | (35) | (38)
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 386,728 | 185,243
Less Than 12 Months, Unrealized Losses | (1,660) | (261)
12 Months or Greater, Estimated Fair Value | 28,087 | 9,144
12 Months or Greater, Unrealized Losses | (50) | (3)
Total Estimated Fair Value | 414,815 | 194,387
Total Unrealized Losses | (1,710) | (264)
Domestic and foreign government agencies debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 10,922 | 12,932
Less Than 12 Months, Unrealized Losses | (82) | (22)
12 Months or Greater, Estimated Fair Value | 7,479 | 0
12 Months or Greater, Unrealized Losses | (24) | 0
Total Estimated Fair Value | 18,401 | 12,932
Total Unrealized Losses | $ (106) | $ (22)

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 investments in debt and equity securities classified as available-for-sale in a continuous loss position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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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