Collaboration and License Agreements (Details) - USD ($)					1 Months Ended	3 Months Ended		6 Months Ended
	May 04, 2019	Sep. 14, 2018	Sep. 11, 2018	May 15, 2018	Oct. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Collaboration agreement
Research and development						$ 21,631,000	$ 16,816,000	$ 39,359,000	$ 28,292,000
Collaboration contract asset						1,687,000		1,687,000		$ 1,958,000
Revenue recognized						$ 2,817,000	1,027,000	$ 5,449,000	2,053,000
Common stock issued						65,309,891		65,309,891		64,693,681
Ono Pharmaceutical Co. Ltd
Collaboration agreement
Aggregate research and development fees payments receivable		$ 20,000,000
Payments for research and development fees					$ 5,000,000
Collaborative arrangement potential additional milestones		40,000,000
Non-refundable upfront payments recorded as deferred revenue		$ 10,000,000
Recognition period of deferred revenue		4 years
Collaborative arrangement annual payments receivable recorded as deferred revenue					$ 5,000,000
Ono Pharmaceutical Co. Ltd | Candidate 2
Collaboration agreement
Percentage of reduction on milestone payments		50.00%
Ono Pharmaceutical Co. Ltd | Maximum | Candidate 1
Collaboration agreement
Aggregate milestone payments		$ 285,000,000
Ono Pharmaceutical Co. Ltd | Maximum | Candidate 2
Collaboration agreement
Aggregate milestone payments		895,000,000
Ono Pharmaceutical Co. Ltd | Collaborative Arrangement
Collaboration agreement
Upfront non-refundable, non-creditable payment receivable		10,000,000
Transaction price of the agreement		30,000,000
Research and development		$ 20,000,000
Sublicense consideration paid						$ 2,000,000		$ 2,000,000
Collaboration contract asset						1,700,000		1,700,000
Amortization of sublicense consideration						200,000		300,000
Revenue recognized						2,500,000		4,100,000
Ono Pharmaceutical Co. Ltd | Collaborative Arrangement | Research services
Collaboration agreement
Revenue recognized						1,700,000		2,700,000
Deferred revenue						10,300,000		10,300,000
Ono Pharmaceutical Co. Ltd | Collaborative Arrangement | Upfront Fee
Collaboration agreement
Revenue recognized						800,000		1,400,000
Deferred revenue classified as current						5,000,000		5,000,000
Ono Pharmaceutical Co. Ltd | Collaborative Arrangement | Other Assets And Deferred Costs
Collaboration agreement
Collaboration contract asset						2,000,000		2,000,000
Ono Pharmaceutical Co. Ltd | Collaborative Arrangement | Minimum
Collaboration agreement
Profits and losses sharing percentage		50.00%
Juno Therapeutics, Inc | Collaborative Arrangement
Collaboration agreement
Revenue recognized						400,000	$ 1,000,000	1,400,000	$ 2,100,000
Research term	4 years
Collaboration agreement expiration date	May 04, 2019
Agreement to terminate upon the receipt of research payment						200,000		200,000
Collaborative additional research payment						0		0
Additional revenue expected to be recognized								0
Juno Therapeutics, Inc | Collaborative Arrangement | Research services
Collaboration agreement
Revenue recognized						200,000		700,000
Juno Therapeutics, Inc | Collaborative Arrangement | Upfront Fee and Equity Premium
Collaboration agreement
Revenue recognized						$ 200,000		$ 700,000
MSK | Amended MSK License
Collaboration agreement
Research and development				$ 5,300,000
Common stock issued				500,000
Upfront cash payment				$ 500,000
Issuance of common stock for license agreements				$ 4,800,000
Gladstone | Gladstone License Agreement
Collaboration agreement
Aggregate milestone payments			$ 1,900,000
Research and development			$ 1,400,000
Common stock issued			100,000
Upfront cash payment			$ 100,000
Issuance of common stock for license agreements			$ 1,300,000