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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, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | $ 421,290 | $ 267,573
Certificates of deposit |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 240 | 960
Commercial paper |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 55,676 | 49,245
Corporate debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 352,897 | 204,436
Domestic and foreign government agencies debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 12,477 | 12,932
Short-term investments |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 215,767 | 224,236
Gross Unrealized Gains | [1] | 1 | 6
Gross Unrealized Losses | [1] | (246) | (159)
Aggregate Estimated Fair Value |  | 215,522 | 224,083
Short-term investments | Certificates of deposit |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 240 | 960
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | 0 | 0
Aggregate Estimated Fair Value |  | $ 240 | $ 960
Short-term investments | Certificates of deposit | Maximum |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Available for sale securities contractual maturity |  | 1 year | 1 year
Short-term investments | Commercial paper |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | $ 55,718 | $ 49,280
Gross Unrealized Gains | [1] | 0 | 3
Gross Unrealized Losses | [1] | (42) | (38)
Aggregate Estimated Fair Value |  | $ 55,676 | $ 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 |  | $ 147,309 | $ 168,548
Gross Unrealized Gains | [1] | 1 | 3
Gross Unrealized Losses | [1] | (181) | (117)
Aggregate Estimated Fair Value |  | $ 147,129 | $ 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 |  | $ 12,500 | $ 5,448
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (23) | (4)
Aggregate Estimated Fair Value |  | $ 12,477 | $ 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 |  | $ 206,236 | $ 43,655
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (468) | (165)
Aggregate Estimated Fair Value |  | 205,768 | 43,490
Long-term investments | Corporate debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 206,236 | 36,149
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (468) | (147)
Aggregate Estimated Fair Value |  | $ 205,768 | $ 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 |  |  | $ 7,506
Gross Unrealized Gains | [1] |  | 0
Gross Unrealized Losses | [1] |  | (18)
Aggregate Estimated Fair Value |  |  | $ 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
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
[1] | (1) Unrealized gains and losses are included in other comprehensive loss.

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