No definition available.

Name:	btrs_LeaseCommitmentsAbstract
Namespace Prefix:	btrs_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Lease Incentive Receivable, Amortization Period

+ References

No definition available.

+ Details

Name:	btrs_LeaseIncentiveReceivableAmortizationPeriod
Namespace Prefix:	btrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee Leasing Arrangements, Operating lease, Monthly Rental Payments

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeaseMonthlyRentalPayments
Namespace Prefix:	btrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Lessee Leasing Arrangements, Operating Lease, Number Of Renewal Options

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeaseNumberOfRenewalOptions
Namespace Prefix:	btrs_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Operating lease rent free period in in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeaseRentFreePeriod
Namespace Prefix:	btrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee Leasing Arrangements, Operating Lease, Size Of Additional Leased Asset Available

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeaseSizeOfAdditionalLeasedAssetAvailable
Namespace Prefix:	btrs_
Data Type:	dtr-types:areaItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee operating Lease waiting period for additional leased asset in in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeaseTermAfterAdditionalLeasedAssetBecomesAvailable
Namespace Prefix:	btrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee Leasing Arrangements, Operating Lease, Term After Which Lease Can Be Terminated

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeaseTermAfterWhichLeaseCanBeTerminated
Namespace Prefix:	btrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee leasing arrangements operating lease termination notice period in in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeaseTerminationAdvanceNoticePeriod
Namespace Prefix:	btrs_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee Leasing Arrangements, Operating Leases, Early Termination Payment

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeasesEarlyTerminationPayment
Namespace Prefix:	btrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessee Leasing Arrangements, Operating Leases, Early Termination Payment, Decrease Per Year

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeasesEarlyTerminationPaymentDecreasePerYear
Namespace Prefix:	btrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessee Leasing Arrangements operating leases early termination payment upon advance notice.

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeasesEarlyTerminationPaymentUponAdvanceNotice
Namespace Prefix:	btrs_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Lessee Leasing Arrangements, Operating Leases, Size of Leased Asset

+ References

No definition available.

+ Details

Name:	btrs_LesseeLeasingArrangementsOperatingLeasesSizeOfLeasedAsset
Namespace Prefix:	btrs_
Data Type:	dtr-types:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495323-112611

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_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of incentive received by lessee from lessor.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=123415192&loc=d3e39896-112707

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 10
-Section 30
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123388062&loc=SL77916639-209961

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 55
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=123386226&loc=d3e41620-112719

Reference 4: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=123415192&loc=d3e40010-112707

+ Details

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

- Definition

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

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

- Definition

This item represents a receivable for an incentive or inducement contractually stipulated between parties to a lease whereby the lessor has committed to provide the entity (lessee) with a cash payment as inducement to enter the lease.

+ References

+ Details

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

- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=123408670&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Definition

Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

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 210.5-02.13)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=btrs_LeaseholdImprovementsFurnitureAndFixturesAndComputerEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LeaseholdImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_CapitalLeaseObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies (FY) (Details) $ in Thousands	1 Months Ended		9 Months Ended		12 Months Ended
Commitments and Contingencies (FY) (Details) $ in Thousands	Jun. 30, 2018 USD ($)	Aug. 31, 2017 USD ($) ft² extensionPeriod	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) ft²
Lease Commitments [Abstract]
Lessee, term of contract		15 years 6 months
Lessee, operating lease, size \| ft²		88,759					88,759
Property, plant and equipment, gross			$ 33,865		$ 32,218	$ 30,455
Incentive from lessor							$ 5,800
Lease incentive receivable							$ 5,800
Lease incentive amortization period							15 years
Lessee, operating lease, size of additional asset \| ft²		61,000
Lessee, operating lease, additional lease asset term		6 years 6 months
Letters of credit outstanding, amount		$ 2,725
Lessee, operating lease, termination term		6 years 6 months
Early termination payment, decrease per year		$ 650
Early termination payment, upon advance notice		$ 3,600
Lease termination, after lease commencement upon notice period		12 years 6 months
Lessee, operating lease, number of renewal options \| extensionPeriod		2
Lessee renewal term		5 years
Operating lease, rent free period	6 months
Operating lease rent expense			3,900	$ 3,900	5,167	5,105	$ 4,226
Purchase obligation					215
Leasehold Improvements, Furniture and Fixtures, and Computer Equipment [Member]
Lease Commitments [Abstract]
Property, plant and equipment, gross							5,700
Improvements [Member]
Lease Commitments [Abstract]
Property, plant and equipment, gross			$ 12,133		$ 12,120	$ 12,071	5,700
Incentive from lessor							$ 5,800
Minimum [Member]
Lease Commitments [Abstract]
Lessee, operating lease, additional lease asset term	181 months
Lessee, operating lease, monthly rental	$ 226
Maximum [Member]
Lease Commitments [Abstract]
Lessee, operating lease, size of additional asset \| ft²		61,000
Lessee, operating lease, additional lease asset term	186 months
Early termination payment		$ 7,500
Lessee, operating lease, monthly rental	$ 281
Capital Lease Obligations [Member] \| Minimum [Member]
Lease Commitments [Abstract]
Debt stated interest rate		5.00%			5.00%
Capital Lease Obligations [Member] \| Maximum [Member]
Lease Commitments [Abstract]
Debt stated interest rate		11.00%			10.60%