Collaboration Agreements - Assumptions Used to Calculate Estimated Fair Value of Success Payment Liability (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Fred Hutchinson Cancer Research Center [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value of common stock	$ 60.66	$ 52.22
Expected volatility	75.00%	75.00%
Fred Hutchinson Cancer Research Center [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Risk free interest rate	1.63%	1.94%
Expected term (years)	6 years 6 months 18 days	6 years 9 months 15 days
Fred Hutchinson Cancer Research Center [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Risk free interest rate	1.91%	2.16%
Expected term (years)	9 years 6 months 18 days	9 years 9 months 15 days
Memorial Sloan Kettering Cancer Center [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair value of common stock	$ 60.66	$ 52.22
Expected volatility	75.00%	75.00%
Memorial Sloan Kettering Cancer Center [Member] \| Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Risk free interest rate	1.65%	1.95%
Expected term (years)	6 years 7 months 21 days	6 years 10 months 21 days
Memorial Sloan Kettering Cancer Center [Member] \| Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Risk free interest rate	1.91%	2.16%
Expected term (years)	9 years 7 months 24 days	9 years 10 months 21 days

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Period the instrument, asset or liability is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ References

+ Details

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- Definition

Risk-free interest rate assumption used in valuing an instrument.

+ References

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- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

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