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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 |

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