Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, unrealized gains	$ 60	$ 11
Available-for-sale debt securities, unrealized losses	(6)	(89)
Cash equivalents, short-term and long-term investments, carrying value	170,390	195,942
Cash equivalents, short-term and long-term investments, fair vale disclosure	170,444	195,864
Money market funds, included in cash equivalents
Debt Securities, Available-for-sale [Line Items]
Cash equivalents at carrying value	50,242	41,434
Money market funds, included in cash equivalents	50,242	41,434
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, unrealized gains	58	3
Available-for-sale debt securities, unrealized losses	(6)	(81)
Total cash equivalents and short-term investments, carrying value	168,791	189,952
Total cash equivalents and short-term investments, fair value	168,843	189,874
Short-term Investments \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	46,695	65,887
Available-for-sale debt securities, unrealized gains	8	2
Available-for-sale debt securities, unrealized losses	(4)	(39)
Available-for-sale debt securities, fair value	46,699	65,850
Short-term Investments \| U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	59,058	53,765
Available-for-sale debt securities, unrealized gains	26	1
Available-for-sale debt securities, unrealized losses	(2)	(8)
Available-for-sale debt securities, fair value	59,082	53,758
Short-term Investments \| Government agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	12,796	28,866
Available-for-sale debt securities, unrealized gains	24	0
Available-for-sale debt securities, unrealized losses	0	(34)
Available-for-sale debt securities, fair value	12,820	28,832
Long-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	1,599	5,990
Available-for-sale debt securities, unrealized gains	2	8
Available-for-sale debt securities, unrealized losses	0	(8)
Available-for-sale debt securities, fair value	1,601	5,990
Long-term Investments \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	1,599	2,001
Available-for-sale debt securities, unrealized gains	2	0
Available-for-sale debt securities, unrealized losses	0	(8)
Available-for-sale debt securities, fair value	$ 1,601	1,993
Long-term Investments \| U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis		3,989
Available-for-sale debt securities, unrealized gains		8
Available-for-sale debt securities, unrealized losses		0
Available-for-sale debt securities, fair value		$ 3,997

Cash Equivalents, Short-term and Long-Term Investments, Carrying Value

No definition available.

Name:	jnce_CashEquivalentsShorttermandLongTermInvestmentsCarryingValue
Namespace Prefix:	jnce_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash Equivalents, Short-term and Long-Term Investments, Fair Vale Disclosure

+ References

No definition available.

+ Details

Name:	jnce_CashEquivalentsShorttermandLongTermInvestmentsFairValeDisclosure
Namespace Prefix:	jnce_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash Equivalents, and Short-term Investments, Carrying Value

+ References

No definition available.

+ Details

Name:	jnce_CashEquivalentsandShorttermInvestmentsCarryingValue
Namespace Prefix:	jnce_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cash Equivalents, and Short-term Investments, Fair Value Disclosure

+ References

No definition available.

+ Details

Name:	jnce_CashEquivalentsandShorttermInvestmentsFairValueDisclosure
Namespace Prefix:	jnce_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ 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 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_CashEquivalentsAtCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=jnce_LongtermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: