Basis of Presentation and Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended			12 Months Ended
	Feb. 28, 2022	Dec. 31, 2021	Oct. 31, 2020	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Revenue recognized in the period from:
Revenue recognized, previously deferred				$ 25,760		$ 72
Revenue from contract with customer				38,078		15,706
Contract assets		$ 3,000		3,000	$ 3,000		$ 3,000
Contract liability		44,351		23,417	44,351		44,351
Deferred revenue		92,068		70,111	92,068	110,037	92,068	$ 110,109
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer				30,892		157
CytomX and Novartis | License and milestone fees
Revenue recognized in the period from:
Revenue recognized, previously deferred						$ 100
KADCYLA | Royalty revenue
Revenue recognized in the period from:
Net proceeds from sale of residual rights to receive royalty payments				4,100
Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer				21,600
Deferred revenue		28,500			28,500		28,500
License agreement upfront payment receivable				45,000
Lilly
Revenue recognized in the period from:
License agreement upfront payment receivable	$ 13,000
Lilly | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer				9,200
Lilly | Upfront payment
Revenue recognized in the period from:
Estimated payments to received	$ 13,000
Deferred revenue				13,000
Viridian | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer				100
Huadong | Upfront payment
Revenue recognized in the period from:
Revenue from contract with customer			$ 40,000
Development milestones | Viridian | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer					3,000
Development milestones | Huadong
Revenue recognized in the period from:
Deferred revenue				6,900
Development milestones | Huadong | License and milestone fees
Revenue recognized in the period from:
Revenue recognized, previously deferred				$ 21,600
Revenue from contract with customer		5,000
Deferred revenue		$ 28,500			$ 28,500		28,500
Development milestones | Huadong | Upfront payment
Revenue recognized in the period from:
Revenue recognized, previously deferred							$ 45,000