Significant Accounting Policies - Schedule of Diluted Net Loss Per Share Attributable to Common Stockholders Due to Anti-Dilutive Effect (Parenthetical) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Success payment	$ 230.3	$ 195.9
Fred Hutchinson Cancer Research Center [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock price per share	$ 60.66	$ 52.22
Memorial Sloan Kettering Cancer Center [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock price per share	$ 60.66	$ 52.22
Estimated shares issued if success payment valuation occurred [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock price per share	$ 60.66
Success payment	205.0
Estimated shares issued if success payment valuation occurred [Member] | Fred Hutchinson Cancer Research Center [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Success payment	125.0
Estimated shares issued if success payment valuation occurred [Member] | Memorial Sloan Kettering Cancer Center [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Success payment	$ 80.0