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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
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Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | $ 407,946 | $ 241,956
Less Than 12 Months, Unrealized Losses | (504) | (321)
12 Months or Greater, Estimated Fair Value | 21,781 | 9,144
12 Months or Greater, Unrealized Losses | (33) | (3)
Total Estimated Fair Value | 429,727 | 251,100
Total Unrealized Losses | (537) | (324)
Commercial paper |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 66,735 | 43,781
Less Than 12 Months, Unrealized Losses | (24) | (38)
12 Months or Greater, Estimated Fair Value | 0 | 0
12 Months or Greater, Unrealized Losses | 0 | 0
Total Estimated Fair Value | 66,735 | 43,781
Total Unrealized Losses | (24) | (38)
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 325,223 | 185,243
Less Than 12 Months, Unrealized Losses | (463) | (261)
12 Months or Greater, Estimated Fair Value | 14,298 | 9,144
12 Months or Greater, Unrealized Losses | (13) | (3)
Total Estimated Fair Value | 339,521 | 194,387
Total Unrealized Losses | (476) | (264)
Domestic and foreign government agencies debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 15,988 | 12,932
Less Than 12 Months, Unrealized Losses | (17) | (22)
12 Months or Greater, Estimated Fair Value | 7,483 | 0
12 Months or Greater, Unrealized Losses | (20) | 0
Total Estimated Fair Value | 23,471 | 12,932
Total Unrealized Losses | $ (37) | $ (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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