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Research Collaboration and License Agreement - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |
Research and development expense | $ 25,705,000 | $ 11,831,000 | $ 7,459,000
Research Collaboration and License Agreement [Member] | PTG-300 [Member] |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |
Upfront payment based on agreement | 250,000 |  |
Potential milestone payment on collaboration arrangement | 250,000 |  |
Research and development expense | 250,000 | $ 0 | $ 0
Maximum [Member] | Research Collaboration and License Agreement [Member] | PTG-300 [Member] |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |
Aggregate milestone payment | $ 128,500,000 |  |

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- Definition The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 730 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 985 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756 + Details
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