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Summary of Investments Classified as Available-For-Sale Securities (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | $ 453,458 | $ 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 |  | 66,735 | 49,245
Corporate debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 363,012 | 204,436
Domestic and foreign government agencies debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Aggregate Estimated Fair Value |  | 23,471 | 12,932
Short-term investments |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 243,372 | 224,236
Gross Unrealized Gains | [1] | 3 | 6
Gross Unrealized Losses | [1] | (176) | (159)
Aggregate Estimated Fair Value |  | 243,199 | 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 |  | $ 66,759 | $ 49,280
Gross Unrealized Gains | [1] | 0 | 3
Gross Unrealized Losses | [1] | (24) | (38)
Aggregate Estimated Fair Value |  | $ 66,735 | $ 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 |  | $ 163,870 | $ 168,548
Gross Unrealized Gains | [1] | 3 | 3
Gross Unrealized Losses | [1] | (131) | (117)
Aggregate Estimated Fair Value |  | $ 163,742 | $ 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 |  | $ 12,503 | $ 5,448
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (21) | (4)
Aggregate Estimated Fair Value |  | $ 12,482 | $ 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 |  | $ 210,620 | $ 43,655
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (361) | (165)
Aggregate Estimated Fair Value |  | 210,259 | 43,490
Long-term investments | Corporate debt securities |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Amortized Cost |  | 199,615 | 36,149
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (345) | (147)
Aggregate Estimated Fair Value |  | $ 199,270 | $ 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 |  | $ 11,005 | $ 7,506
Gross Unrealized Gains | [1] | 0 | 0
Gross Unrealized Losses | [1] | (16) | (18)
Aggregate Estimated Fair Value |  | $ 10,989 | $ 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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