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Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | $ 446,872 | $ 460,252
Less Than 12 Months, Unrealized Losses | (1,959) | (1,777)
12 Months or Greater, Estimated Fair Value | 138,935 | 35,566
12 Months or Greater, Unrealized Losses | (850) | (74)
Total Estimated Fair Value | 585,807 | 495,818
Total Unrealized Losses | (2,809) | (1,851)
Commercial paper |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 37,157 | 62,602
Less Than 12 Months, Unrealized Losses | (54) | (35)
12 Months or Greater, Estimated Fair Value | 0 | 0
12 Months or Greater, Unrealized Losses | 0 | 0
Total Estimated Fair Value | 37,157 | 62,602
Total Unrealized Losses | (54) | (35)
Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 398,814 | 386,728
Less Than 12 Months, Unrealized Losses | (1,803) | (1,660)
12 Months or Greater, Estimated Fair Value | 138,935 | 28,087
12 Months or Greater, Unrealized Losses | (850) | (50)
Total Estimated Fair Value | 537,749 | 414,815
Total Unrealized Losses | (2,653) | (1,710)
Domestic and foreign government agencies debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Less Than 12 Months, Estimated Fair Value | 10,901 | 10,922
Less Than 12 Months, Unrealized Losses | (102) | (82)
12 Months or Greater, Estimated Fair Value | 0 | 7,479
12 Months or Greater, Unrealized Losses | 0 | (24)
Total Estimated Fair Value | 10,901 | 18,401
Total Unrealized Losses | $ (102) | $ (106)

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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 No definition available. + 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://fasb.org/us-gaap/role/ref/legacyRef -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 that have been in a continuous loss position for less than twelve months. + References No definition available. + Details
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