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Convertible Senior Notes - Additional Information (Detail) $ / shares in Units, $ in Thousands |  | 12 Months Ended
--- | --- | ---
May 02, 2017 USD ($) | Dec. 31, 2017 USD ($) d $ / shares | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Debt Instrument [Line Items] |  |  |  |
Convertible senior notes principal amount |  | $ 517,500 |  |
Proceeds from Senior Convertible Notes |  | $ 502,781 | $ 0 | $ 0
2.25% Convertible Senior Notes due 2024 |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Convertible senior notes principal amount | $ 517,500 |  |  |
Convertible senior notes interest percentage | 2.25% |  |  |
Convertible senior notes convertible latest date |  | May 15, 2024 |  |
Convertible senior notes interest rate | 2.25% |  |  |
Proceeds from Senior Convertible Notes | $ 502,800 |  |  |
Threshold common stock trading days | d |  | 20 |  |
Threshold consecutive common stock trading days | d |  | 30 |  |
Threshold percentage of common stock price trigger |  | 130.00% |  |
Minimum percentage of common stock price trigger |  | 98.00% |  |
Conversion Observation Period |  | 25 days |  |
Convertible senior notes convertible in to shares |  | 0.00131711 |  |
Convertible senior notes conversion price | $ / shares |  | $ 75.92 |  |
Market Price of Common stock | $ / shares |  | $ 53.28 |  |
Convertible senior note premium |  | 42.50% |  |
Convertible senior notes redemption rate |  | 100.00% |  |
Convertible senior note carrying value of the liability component |  | $ 368,300 |  |
Convertible senior note assumed borrowing rate |  | 7.50% |  |
Convertible senior note carrying value of the equity component |  | $ 149,200 |  |
Convertible senior notes |  | 7 years |  |
Transaction cost related to issuance of convertible senior note |  | $ 14,700 |  |

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- Definition The minimum percentage of the product of the common stock trading price multiplied by the conversion rate during the measurement period as compared to the principal note amount that triggers a conversion option on the notes, if it falls below this percentage. + References No definition available. + Details
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- Definition The rate of interest that was being paid on the original debt issue that is being converted in the noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586 + Details
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