Organization and Summary of Significant Accounting Policies (Detail)		3 Months Ended	9 Months Ended		12 Months Ended
	Apr. 13, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2020 USD ($)	Oct. 05, 2015 USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]
Cash and investments in marketable securities			$ 1,153,721,000		$ 1,603,981,000
Accounting Standards Update [Extensible List]					us-gaap:AccountingStandardsUpdate201911Member
Tax benefit elimination in continuing operations and tax provision in other comprehensive income (loss)		$ 200,000		$ (1,300,000)
Number of business segments | segment			1
Accounts receivable			$ 42,925,000		$ 36,802,000
Repayments of senior notes			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 notes	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			13,000,000.0		12,800,000
BMS Collaboration Agreement
Organization And Summary Of Significant Accounting Policies [Line Items]
Accounts receivable			$ 29,900,000		$ 24,000,000.0