Organization - Additional Information (Detail) (USD $)
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1 Months Ended | 2 Months Ended | ||||||
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Feb. 28, 2014
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May 31, 2014
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Oct. 31, 2013
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Jul. 31, 2014
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Dec. 31, 2014
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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Organization [Line Items] | ||||||||
Accumulated deficit | $ 48,491,000 | $ 41,341,000 | ||||||
Cash and cash equivalents | 293,579,000 | 176,677,000 | 124,860,000 | 0 | ||||
Collaboration License Revenue [Member] | Roche [Member] | ||||||||
Organization [Line Items] | ||||||||
Upfront Payment pursuant to License Agreement | 30,000,000 | |||||||
Collaborative Arrangement [Member] | Roche [Member] | ||||||||
Organization [Line Items] | ||||||||
Milestone Payment Received, Clinical Milestone | 15,000,000 | |||||||
Underwritten Public And Over-Allotment Offering [Member] | Ordinary Shares | ||||||||
Organization [Line Items] | ||||||||
Ordinary shares sold pursuant to underwritten public offering (in shares) | 4,177,079 | 5,462,500 | ||||||
Net proceeds from issuance of ordinary shares in underwritten public offering | $ 117,400,000 |