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Summary of Investments Classified as Available-For-Sale Securities (Detail) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | $ 591,706 | $ 508,578
Commercial paper |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 37,157 | 75,362
Corporate debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 543,648 | 414,815
Domestic and foreign government agencies debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 10,901 | 18,401
Short-term investments |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | 343,662 | 261,675
Gross Unrealized Gains | [1] | 0 | 1
Gross Unrealized Losses | [1] | (1,344) | (459)
Aggregate Estimated Fair Value |  | 342,318 | 261,217
Short-term investments | Commercial paper |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | 37,211 | 75,396
Gross Unrealized Gains | [1] | 0 | 1
Gross Unrealized Losses | [1] | (54) | (35)
Aggregate Estimated Fair Value |  | $ 37,157 | $ 75,362
Short-term investments | Commercial paper | Maximum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 1 year | 1 year
Short-term investments | Corporate debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | $ 300,451 | $ 178,776
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (1,243) | (400)
Aggregate Estimated Fair Value |  | $ 299,208 | $ 178,376
Short-term investments | Corporate debt securities | Maximum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 1 year | 1 year
Short-term investments | Domestic and foreign government agencies debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | $ 6,000 | $ 7,503
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (47) | (24)
Aggregate Estimated Fair Value |  | $ 5,953 | $ 7,479
Short-term investments | Domestic and foreign government agencies debt securities | Maximum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 1 year | 1 year
Long-term investments |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | $ 250,853 | $ 248,753
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (1,465) | (1,392)
Aggregate Estimated Fair Value |  | 249,388 | 247,361
Long-term investments | Corporate debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | 245,850 | 237,749
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (1,410) | (1,310)
Aggregate Estimated Fair Value |  | $ 244,440 | $ 236,439
Long-term investments | Corporate debt securities | Minimum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 1 year | 1 year
Long-term investments | Corporate debt securities | Maximum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 2 years | 2 years
Long-term investments | Domestic and foreign government agencies debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | $ 5,003 | $ 11,004
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (55) | (82)
Aggregate Estimated Fair Value |  | $ 4,948 | $ 10,922
Long-term investments | Domestic and foreign government agencies debt securities | Minimum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 1 year | 1 year
Long-term investments | Domestic and foreign government agencies debt securities | Maximum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 2 years | 2 years
[1] | Unrealized gains and losses are included in other comprehensive loss.

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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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