Significant Agreements - Juno Therapeutics Collaboration Agreement (Details) $ in Thousands	1 Months Ended				3 Months Ended								10 Months Ended	12 Months Ended			32 Months Ended
	Jul. 31, 2017 USD ($)	Mar. 31, 2017 USD ($) item	May 31, 2016 USD ($)	May 31, 2015 USD ($) item	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)
Collaboration and other research and development revenues					$ 3,667	$ 6,282	$ 3,097	$ 682	$ 898	$ 962	$ 3,388	$ 805		$ 13,728	$ 6,053	$ 1,629
Advance payment on future research and development activities					1,864				1,662				$ 1,864	1,864	1,662		$ 1,864
Research and development														83,159	56,979	18,846
Deferred revenue, long-term					94,725				26,000				94,725	94,725	26,000		94,725
Accounts receivable					679				88				679	$ 679	88		679
Allergan
Number of additional licenses | item														2
One time payment to acquire the license					25,000								25,000	$ 25,000			25,000
Potential development milestone payments					8,000								8,000	$ 8,000			8,000
Number of days in which payment to be made														45 days
Allergan | Maximum
Potential development milestone payments					42,000								42,000	$ 42,000			42,000
Potential regulatory and commercial milestone payments					75,000								75,000	75,000			75,000
Potential commercial milestone payments					90,000								90,000	90,000			90,000
Collaboration and License Agreement | Juno Therapeutics
Number of research areas | item				3
Agreement term				5 years
Extensions | item				2
Extension period				1 year
Upfront fee received				$ 25,000
Potential research and development funding				22,000
Potential fee receivable for each gene target licensed				$ 2,500
Number of additional licenses | item				3
Potential development milestone payments				$ 77,500
Potential regulatory milestone payments				80,000
Potential commercial milestone payments				$ 75,000
Milestone payment received under license agreement	$ 2,500		$ 2,500
Next possible milestone payment					2,500								2,500	2,500			2,500
Number of options to purchase Development and Commercialization License | item				3
Number of deliverables | item				6
Number of accounting units | item				11
Number of accounting units for Development and Commercialization License | item				3
Number of accounting units for discount deliverables | item				6
Allocable arrangement consideration - upfront payment				$ 25,000
Allocable arrangement consideration - research support				20,000
Allocable arrangement consideration - specialized material costs				2,000
Allocable arrangement consideration - aggregate arrangement consideration				47,000
Allocable arrangement consideration - R&D Services Unit of Accounting				16,700
Allocable arrangement consideration - Development and Commercialization License for the first research area				9,300
Allocable arrangement consideration - Development and Commercialization License for the second research area				15,400
Allocable arrangement consideration - Development and Commercialization License for the third research area				200
Allocable arrangement consideration - first Discount Deliverable for the first research area				700
Allocable arrangement consideration - second Discount Deliverable for the first research area				400
Allocable arrangement consideration - third Discount Deliverable for the first research area				200
Allocable arrangement consideration - first Discount Deliverable for the second research area				2,000
Allocable arrangement consideration - second Discount Deliverable for the second research area				1,300
Allocable arrangement consideration - third Discount Deliverable for the second research area				800
Allocable arrangement consideration -JRC Deliverable				$ 0
Collaboration and other research and development revenues														4,900	5,700	$ 1,600	12,200
Research and development														500	500
Deferred revenue					26,400				26,000				26,400	26,400	26,000		26,400
Accounts receivable					500				$ 0				500	500	$ 0		500
Strategic Alliance | Allergan
Agreement term		7 years
Number of Collaboration Development Programs | item		5
Number of Committee Members | item		3
Extension period		1 year
Upfront fee received		$ 90,000
Option exercise price per CDP		15,000						15,000
Extension fee for Initial Option Period		5,000						5,000
Option exercise price for development and commercialization license		22,500						22,500
Allocable arrangement consideration - aggregate arrangement consideration		$ 90,000						$ 90,000
Collaboration and other research and development revenues													8,800	8,800
Research and development														14,100
Deferred revenue					81,200								81,200	81,200			81,200
Deferred revenue, long-term					$ 68,300								$ 68,300	$ 68,300			$ 68,300
Contract termination notice														90 days
Strategic Alliance | Allergan | Maximum
Extension period		10 years