Summary Of Significant Accounting Policies - Warrants to Purchase Convertible Preferred Stock (Details)	1 Months Ended		3 Months Ended								10 Months Ended	12 Months Ended			20 Months Ended	32 Months Ended
	Jul. 31, 2017 USD ($)	May 31, 2016 USD ($)	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 ($)	Dec. 31, 2017 USD ($)
Recent Accounting Pronouncements
Number of Revenue Agreements | item												3
Collaboration and other research and development revenues			$ 3,667,000	$ 6,282,000	$ 3,097,000	$ 682,000	$ 898,000	$ 962,000	$ 3,388,000	$ 805,000		$ 13,728,000	$ 6,053,000	$ 1,629,000
Adjustment to valuation allowance												19,000,000
ASU 2016-09
Recent Accounting Pronouncements
Adjustment to valuation allowance												179,000
Juno Therapeutics | Collaboration and License Agreement
Recent Accounting Pronouncements
Collaboration and other research and development revenues												4,900,000	$ 5,700,000	$ 1,600,000		$ 12,200,000
Milestone payment received under license agreement	$ 2,500,000	$ 2,500,000
Cystic Fibrosis Foundation Therapeutics, Inc. | Development award agreement
Recent Accounting Pronouncements
Collaboration and other research and development revenues															$ 300,000
Allergan | Strategic Alliance
Recent Accounting Pronouncements
Collaboration and other research and development revenues											$ 8,800,000	$ 8,800,000