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Investments - Summary of Investments Classified as AFS Debt Securities (Detail) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2019 | Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | $ 702,861 | $ 725,227
Commercial paper |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 116,490 | 94,572
Corporate debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 419,293 | 544,978
Securities of government-sponsored entities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 89,713 | 85,677
Short-term investments |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | 478,553 | 510,889
Gross Unrealized Gains | [1] | 543 | 8
Gross Unrealized Losses | [1] | (103) | (1,698)
Aggregate Estimated Fair Value |  | 478,993 | 509,199
Short-term investments | Commercial paper |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | 116,446 | 94,617
Gross Unrealized Gains | [1] | 45 |
Gross Unrealized Losses | [1] | (1) | (45)
Aggregate Estimated Fair Value |  | $ 116,490 | $ 94,572
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 |  | $ 302,876 | $ 395,385
Gross Unrealized Gains | [1] | 299 |
Gross Unrealized Losses | [1] | (96) | (1,598)
Aggregate Estimated Fair Value |  | $ 303,079 | $ 393,787
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 | Securities of government-sponsored entities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | $ 59,231 | $ 20,887
Gross Unrealized Gains | [1] | 199 | 8
Gross Unrealized Losses | [1] | (6) | (55)
Aggregate Estimated Fair Value |  | $ 59,424 | $ 20,840
Short-term investments | Securities of government-sponsored entities | 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 |  | $ 145,418 | $ 216,270
Gross Unrealized Gains | [1] | 1,101 | 228
Gross Unrealized Losses | [1] | (16) | (470)
Aggregate Estimated Fair Value |  | 146,503 | 216,028
Long-term investments | Corporate debt securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | 115,454 | 151,594
Gross Unrealized Gains | [1] | 776 | 66
Gross Unrealized Losses | [1] | (16) | (469)
Aggregate Estimated Fair Value |  | $ 116,214 | $ 151,191
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 | Securities of government-sponsored entities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Amortized Cost |  | $ 29,964 | $ 64,676
Gross Unrealized Gains | [1] | 325 | 162
Gross Unrealized Losses | [1] |  | (1)
Aggregate Estimated Fair Value |  | $ 30,289 | $ 64,837
Long-term investments | Securities of government-sponsored entities | Minimum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 1 year | 1 year
Long-term investments | Securities of government-sponsored entities | Maximum |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 2 years | 2 years
[1] | Unrealized gains and losses, net of tax, are included in other comprehensive income (loss)

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