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Summary of Investments Classified as Available-For-Sale Securities (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | $ 508,578 | $ 267,573
Certificates of deposit |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 0 | 960
Commercial paper |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 75,362 | 49,245
Corporate debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 414,815 | 204,436
Domestic and foreign government agencies debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 18,401 | 12,932
Short-term investments |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 261,675 | 224,236
Gross Unrealized Gains | [1] | 1 | 6
Gross Unrealized Losses | [1] | (459) | (159)
Aggregate Estimated Fair Value |  | 261,217 | 224,083
Short-term investments | Certificates of deposit |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  |  | 960
Gross Unrealized Gains | [1] |  | 0
Gross Unrealized Losses | [1] |  | 0
Aggregate Estimated Fair Value |  |  | $ 960
Short-term investments | Certificates of deposit | Maximum |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  |  | 1 year
Short-term investments | Commercial paper |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 75,396 | $ 49,280
Gross Unrealized Gains | [1] | 1 | 3
Gross Unrealized Losses | [1] | (35) | (38)
Aggregate Estimated Fair Value |  | $ 75,362 | $ 49,245
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 |  | $ 178,776 | $ 168,548
Gross Unrealized Gains | [1] | 0 | 3
Gross Unrealized Losses | [1] | (400) | (117)
Aggregate Estimated Fair Value |  | $ 178,376 | $ 168,434
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 |  | $ 7,503 | $ 5,448
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (24) | (4)
Aggregate Estimated Fair Value |  | $ 7,479 | $ 5,444
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 |  | $ 248,753 | $ 43,655
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (1,392) | (165)
Aggregate Estimated Fair Value |  | 247,361 | 43,490
Long-term investments | Corporate debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 237,749 | 36,149
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (1,310) | (147)
Aggregate Estimated Fair Value |  | $ 236,439 | $ 36,002
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 |  | $ 11,004 | $ 7,506
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (82) | (18)
Aggregate Estimated Fair Value |  | $ 10,922 | $ 7,488
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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