License and Collaboration Agreement - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred revenue - related party
Balance at Beginning of Period	$ 8,223	$ 8,223
Balance at End of Period		17,738	$ 8,223
Payable to collaboration partner - related party
Balance at Beginning of Period	1,061	1,061
Balance at End of Period		1,262	1,061
Revenue recognized from contract liability balance at beginning of period		1,600	23,500	$ 0
Costs to obtain or fulfill the contract that were capitalized		0	0	0
Janssen Biotech, Inc. | License and Collaborative Revenue
Payable to collaboration partner - related party
Revenue recognized from contract liability balance at beginning of period		200	30,900	20,100
Increase (decrease) in transaction price	1,600	(9,400)
Janssen Biotech, Inc. | License and Collaboration Agreement
Receivable from collaboration partner - related party
Balance at Beginning of Period	2,042	2,042	1,816
Additions		36,837	6,665
Deductions		(32,924)	(6,439)
Balance at End of Period		5,955	2,042	1,816
Contract asset - related party
Contract asset - related party, Balance at Beginning of Period	2,545	2,545
Contract asset - related party, Additions		800	2,545
Contract asset - related party, Deductions		(2,545)
Contract asset - related party, Balance at End of Period		800	2,545
Deferred revenue - related party
Balance at Beginning of Period	8,223	8,223	31,752
Additions		42,456	7,296
Deductions		(9,149)	(30,825)	(19,000)
Balance at End of Period		41,530	8,223	31,752
Payable to collaboration partner - related party
Balance at Beginning of Period	$ 1,061	1,061
Additions		1,468	1,574
Deductions		(1,267)	(513)
Balance at End of Period		$ 1,262	1,061
Increase (decrease) in transaction price			$ 52,200	$ 59,000