Organization and Summary of Significant Accounting Policies (Detail)	3 Months Ended
	Mar. 31, 2021 USD ($) segment	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]
Cash and investments in marketable securities	$ 1,133,409,000	$ 1,198,558,000
Number of business segment | segment	1
Accounts receivable	$ 29,156,000	38,889,000
Allowance for credit loss	0	0
Aggregate offering price	300,000,000.0
NKTR - 181
Organization And Summary Of Significant Accounting Policies [Line Items]
Write off of prepayment			$ 19,700,000
Purchase commitment, remaining minimum amount committed			$ 25,500,000
BMS Collaboration Agreement
Organization And Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	26,200,000	38,700,000
Collaboration Partner
Organization And Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	$ 2,900,000	$ 200,000