Lease additional term.

No definition available.

Name:	kymr_LeaseAdditionalTerm
Namespace Prefix:	kymr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lease commencement date.

+ References

No definition available.

+ Details

Name:	kymr_LeaseCommencementDate
Namespace Prefix:	kymr_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Lease term.

+ References

No definition available.

+ Details

Name:	kymr_LeaseTerm
Namespace Prefix:	kymr_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lease termination date.

+ References

No definition available.

+ Details

Name:	kymr_LeaseTerminationDate
Namespace Prefix:	kymr_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Tenant incentive allowance receivable.

+ References

No definition available.

+ Details

Name:	kymr_TenantIncentiveAllowanceReceivable
Namespace Prefix:	kymr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

The adjustment to reduce the value of existing agreements that specify the lessee's rights to use the leased property. This expense is charged when the estimates of future profits generated by the leased property are reduced.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228

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

+ Details

Name:	us-gaap_ImpairmentOfLeasehold
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Date which lease or group of leases is set to expire, in CCYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_LeaseExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Net cash outflow or inflow from monetary allowance granted by the landlord to a tenant to entice tenant to move into landlords building which will enable the tenant to prepare the leased premises for tenants occupancy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585

+ Details

Name:	us-gaap_PaymentsForProceedsFromTenantAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of an asset, typically cash, provided to a counterparty to provide certain assurance of performance by the entity pursuant to the terms of a written or oral agreement, such as a lease.

+ 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 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Carrying amount as of the balance sheet date of improvements having a life longer than one year that were made for the benefit of one or more tenants.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

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

- Details

Name:	srt_StatementGeographicalAxis=kymr_CambridgeMassachusettsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=kymr_NoncancelableFacilityLeaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=kymr_FacilitySubleaseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=kymr_WatertownMassachusettsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Leases - Additional Information (Details) $ in Millions	1 Months Ended			12 Months Ended
Leases - Additional Information (Details) $ in Millions	Mar. 31, 2020 USD ($)	Oct. 31, 2019 USD ($) ft²	Apr. 30, 2019 ft²	Sep. 30, 2020 USD ($)	Oct. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Feb. 28, 2018 ft²
Cambridge, Massachusetts \| Noncancelable Facility Lease Agreement
Lessee Lease Description [Line Items]
Area of office space \| ft²							9,836
Reduction of right-of-use asset and liability due to lease modification	$ 2.2
Lease termination date	Jul. 31, 2020
Cambridge, Massachusetts \| Noncancelable Facility Lease Agreement \| Other Assets \| Letter of Credit
Lessee Lease Description [Line Items]
Security deposit						$ 0.2
Cambridge, Massachusetts \| Noncancelable Facility Lease Agreement \| Prepaid Expenses and Other Current Assets \| Letter of Credit
Lessee Lease Description [Line Items]
Security deposit				$ 0.2
Cambridge, Massachusetts \| Noncancelable Facility Lease Agreement \| Other Current Assets \| Subsequent Events \| Letter of Credit
Lessee Lease Description [Line Items]
Security deposit					$ 0.2
Cambridge, Massachusetts \| Facility Sublease Agreement
Lessee Lease Description [Line Items]
Area of office space \| ft²			1,471
Lease commencement date			Jun. 24, 2019
Watertown, Massachusetts \| Noncancelable Facility Lease Agreement
Lessee Lease Description [Line Items]
Area of office space \| ft²		34,522
Lease term		120 months
Lease expiration date		Mar. 31, 2030
Lease additional term		5 years
Tenant incentive allowance receivable		$ 5.5
Cash reimbursements received from landlord				5.3
Watertown, Massachusetts \| Noncancelable Facility Lease Agreement \| Other Assets \| Letter of Credit
Lessee Lease Description [Line Items]
Security deposit				1.6
Watertown, Massachusetts \| Noncancelable Facility Lease Agreement \| Other Current Assets
Lessee Lease Description [Line Items]
Tenant improvement receivable				$ 0.2