XML 56 R41.htm IDEA: XBRL DOCUMENT
v3.20.2

Loan and Security Agreement - Additional Information (Details) |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Feb. 28, 2020 USD ($) Installment $ / shares shares | Aug. 31, 2017 USD ($) Installment | Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) | Oct. 01, 2018 shares | Jul. 31, 2018 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest payable |  |  | $ 200,000 |  | $ 200,000 |  | $ 400,000 |  |
Debt, non-current |  |  | 24,411,000 |  | 24,411,000 |  | 8,876,000 |  |
Loan and accretion of debt discount |  |  | 100,000 | $ 79,000 | 300,000 | $ 254,000 |  |  |
Interest expense, debt |  |  | $ 700,000 | $ 300,000 | $ 1,700,000 | $ 900,000 |  |  |
Average interest rate |  |  | 8.07% | 8.64% | 8.08% | 8.80% |  |  |
Two Thousand Seventeen Warrants |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Estimated fair value portion of preferred stock |  |  |  |  |  |  |  |  | $ 300,000
Series E Redeemable Convertible Preferred Stock |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Warrant to purchase stock | shares |  |  |  |  |  |  |  | 46,359 |
August 2017 Loan | Floating Rate |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest charges on loan |  | 7.39% |  |  |  |  |  |  |
August 2017 Loan | LIBOR |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest charges on loan |  | 6.40% |  |  |  |  |  |  |
Description of interest charges on loan |  |  |  |  | the sum of the 30-day U.S. Dollar London Interbank Offered Rate (“LIBOR”) plus 6.40%. |  |  |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Warrant to purchase stock | shares | 81,257 |  |  |  |  |  |  |  |
Exercise price per share | $ / shares | $ 9.23 |  |  |  |  |  |  |  |
Estimated fair value upon issuance of warrants | $ 600,000 |  |  |  |  |  |  |  |
Debt, non-current |  |  | $ 25,000,000.0 |  | $ 25,000,000.0 |  |  |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | August 2017 Loan |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Borrowed amount |  | $ 15,000,000.0 |  |  |  |  |  |  |
Debt outstanding amount |  |  |  |  |  |  | $ 10,000,000.0 |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | August 2017 Loan | March 2019 |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Number of monthly installments | Installment |  | 30 |  |  |  |  |  |  |
Payment terms, description |  |  |  |  | The loan was due in 30 monthly installments from March 2019 |  |  |  |
Repayment installment, start date, month and year |  | 2019-03 |  |  |  |  |  |  |
Repayment installment, end date, month and year |  | 2021-08 |  |  |  |  |  |  |
Frequency of interest-only payments |  |  |  |  | monthly payments |  |  |  |
Interest-only payments, maturity month and year |  | 2019-03 |  |  |  |  |  |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | Term A Loan |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Borrowed amount | 25,000,000.0 |  |  |  |  |  |  |  |
Term loan maximum borrowing capacity | 25,000,000.0 |  |  |  |  |  |  |  |
Repayment of outstanding principal amount | $ 9,600,000 |  |  |  |  |  |  |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | Term Loans |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Number of monthly installments | Installment | 24 |  |  |  |  |  |  |  |
Payment terms, description |  |  |  |  | The Term Loans mature on March 1, 2024 (the “Maturity Date”) and will be interest-only through March 1, 2022, followed by 24 equal monthly payments of principal and interest. |  |  |  |
Interest charges on loan | 8.07% |  |  |  |  |  |  |  |
Description of interest charges on loan |  |  |  |  | The Term Loans will bear interest at a floating per annum rate equal to the greater of (i) 8.07% or (ii) the sum of (a) the greater of (1) the thirty (30) day U.S. LIBOR rate reported in the Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue or (2) 1.67%, plus (b) 6.40%. |  |  |  |
Debt instrument, maturity date | Mar. 01, 2024 |  |  |  |  |  |  |  |
Debt instrument interest only payments maturity date | Mar. 01, 2022 |  |  |  |  |  |  |  |
Floor percent of thirty days libor | 1.67% |  |  |  |  |  |  |  |
Final payment fee percentage | 3.83% |  |  |  |  |  |  |  |
Interest payable | $ 1,000,000.0 |  |  |  |  |  |  |  |
Principal amount Prepayment amount percentage to be paid before first anniversary | 3.00% |  |  |  |  |  |  |  |
Principal amount Prepayment amount percentage to be paid between first and second anniversary | 2.00% |  |  |  |  |  |  |  |
Principal amount Prepayment amount percentage to be paid to be paid thereafter and prior to maturity | 1.00% |  |  |  |  |  |  |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | Term Loans | Maximum |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Additional borrowing amount | $ 5,000,000.0 |  |  |  |  |  |  |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | Term Loans | LIBOR |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Interest charges on loan | 6.40% |  |  |  |  |  |  |  |
Loan and Security Agreement | Oxford Finance LLC and Silicon Valley Bank | Term B Loan | Minimum |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |
Upfront payment received | $ 50,000,000.0 |  |  |  |  |  |  |  |

X
- Definition Debt instrument, final payment fee percentage. + References No definition available. + Details
Name: | stro_DebtInstrumentFinalPaymentFeePercentage
Namespace Prefix: | stro_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument interest only payments maturity date. + References No definition available. + Details
Name: | stro_DebtInstrumentInterestOnlyPaymentsMaturityDate
Namespace Prefix: | stro_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument, interest only payments maturity month and year. + References No definition available. + Details
Name: | stro_DebtInstrumentInterestOnlyPaymentsMaturityMonthAndYear
Namespace Prefix: | stro_
Data Type: | xbrli:gYearMonthItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument, number of monthly installments. + References No definition available. + Details
Name: | stro_DebtInstrumentNumberOfMonthlyInstallments
Namespace Prefix: | stro_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument principal amount prepayment percentage to be paid before first anniversary. + References No definition available. + Details
Name: | stro_DebtInstrumentPrincipalAmountPrepaymentPercentageToBePaidBeforeFirstAnniversary
Namespace Prefix: | stro_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument principal amount prepayment percentage to be paid between first and second anniversary. + References No definition available. + Details
Name: | stro_DebtInstrumentPrincipalAmountPrepaymentPercentageToBePaidBetweenFirstAndSecondAnniversary
Namespace Prefix: | stro_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument principal amount prepayment percentage to be paid thereafter and prior to maturity. + References No definition available. + Details
Name: | stro_DebtInstrumentPrincipalAmountPrepaymentPercentageToBePaidThereafterAndPriorToMaturity
Namespace Prefix: | stro_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument, repayment installment end date of month and year. + References No definition available. + Details
Name: | stro_DebtInstrumentRepaymentInstallmentEndDateOfMonthAndYear
Namespace Prefix: | stro_
Data Type: | xbrli:gYearMonthItemType
Balance Type: | na
Period Type: | duration

X
- Definition Debt instrument repayment installment start date of month and year. + References No definition available. + Details
Name: | stro_DebtInstrumentRepaymentInstallmentStartDateOfMonthAndYear
Namespace Prefix: | stro_
Data Type: | xbrli:gYearMonthItemType
Balance Type: | na
Period Type: | duration

X
- Definition Estimated fair value portion of issuance of redeemable convertible preferred stock. + References No definition available. + Details
Name: | stro_EstimatedFairValuePortionOfIssuanceOfRedeemableConvertiblePreferredStock
Namespace Prefix: | stro_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Estimated fair value upon issuance of warrants. + References No definition available. + Details
Name: | stro_EstimatedFairValueUponIssuanceOfWarrants
Namespace Prefix: | stro_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Floor percent of thirty days libor. + References No definition available. + Details
Name: | stro_FloorPercentOfThirtyDaysLibor
Namespace Prefix: | stro_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line of credit facility additional borrowing amount. + References No definition available. + Details
Name: | stro_LineOfCreditFacilityAdditionalBorrowingAmount
Namespace Prefix: | stro_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Upfront payment received. + References No definition available. + Details
Name: | stro_UpfrontPaymentReceived
Namespace Prefix: | stro_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28541-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 + Details
Name: | us-gaap_AmortizationOfDebtDiscountPremium
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Exercise price per share or per unit of warrants or rights outstanding. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644 + Details
Name: | us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644 + Details
Name: | us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400 + Details
Name: | us-gaap_DebtInstrumentFaceAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Description of the frequency of periodic payments (monthly, quarterly, annual). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823 + Details
Name: | us-gaap_DebtInstrumentFrequencyOfPeriodicPayment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The average effective interest rate during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateDuringPeriod
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_DebtInstrumentInterestRateTerms
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(2)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_DebtInstrumentMaturityDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(3)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823 + Details
Name: | us-gaap_DebtInstrumentPaymentTerms
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the aggregate of total long-term debt, including current maturities and short-term debt. + References No definition available. + Details
Name: | us-gaap_DebtLongtermAndShorttermCombinedAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of the cost of borrowed funds accounted for as interest expense for debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6036836-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 + Details
Name: | us-gaap_InterestExpenseDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of interest payable on debt, including, but not limited to, trade payables. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15(5)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15(a)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 + Details
Name: | us-gaap_InterestPayableCurrentAndNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
Name: | us-gaap_LongTermDebtNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(f)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 + Details
Name: | us-gaap_RepaymentsOfLinesOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_ClassOfWarrantOrRightAxis=stro_TwoThousandSeventeenWarrantsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_StatementClassOfStockAxis=stro_SeriesERedeemableConvertiblePreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=stro_AugustTwoThousandSeventeenLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=stro_FloatingRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_TypeOfArrangementAxis=stro_LoanAndSecurityAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=stro_OxfordFinanceLimitedLiabilityCompanyAndSiliconValleyBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=stro_MarchTwoThousandNineteenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=stro_TermALoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=stro_TermLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=stro_TermBLoanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |