Organization and Summary of Significant Accounting Policies - Additional Information (Detail)		3 Months Ended	6 Months Ended
	Apr. 13, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Segment	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Oct. 05, 2015 USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]
Cash and investments in marketable securities			$ 1,210,563,000			$ 1,603,981,000
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201911Member
Tax provision on unrealized gain on available-for-sale securities, offset by benefit in continuing operations		$ 400,000		$ 1,500,000
Number of business segment | Segment			1
Accounts receivable for expense reimbursements from collaboration partner			$ 47,245,000			36,802,000
Contract asset			25,000,000.0
Repayments of Senior Debt			250,000,000	$ 0
7.75% Senior Secured Notes Due October 2020 | Senior Notes
Organization And Summary Of Significant Accounting Policies [Line Items]
Repayments of long term debt, including interest	$ 254,800,000
Debt instrument, face amount							$ 250,000,000.0
Senior secured notes, interest rate							7.75%
Repayments of Senior Debt	250,000,000.0
Cash paid for interest	$ 4,800,000
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
Collaboration Partner
Organization And Summary Of Significant Accounting Policies [Line Items]
Accounts receivable for expense reimbursements from collaboration partner			13,300,000			12,800,000
BMS Collaboration Agreement
Organization And Summary Of Significant Accounting Policies [Line Items]
Accounts receivable for expense reimbursements from collaboration partner			$ 33,900,000			$ 24,000,000.0