Revenue From Contracts With Customers, Obligation Upon Sale, Percentage

No definition available.

Name:	btrs_RevenueFromContractsWithCustomersObligationUponSalePercentage
Namespace Prefix:	btrs_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Revenue From Contracts With Customers, Payment Period

+ References

No definition available.

+ Details

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

- Definition

Revenue From Contracts With Customers, Practical Expedient, Original Expected Duration

+ References

No definition available.

+ Details

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

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130556-203045

+ Details

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

- Definition

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130556-203045

+ Details

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of remaining performance obligation to total remaining performance obligation not recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130556-203045

+ Details

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

- Details

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis=2021-10-01
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis=2021-12-31
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis=2022-12-31
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis=2023-12-31
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:

Revenue from Contracts with Customers (FY) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Revenue, Performance Obligation [Abstract]
Revenue, payment period					30 days
Contract with customer, liability, revenue recognized	$ 3.5	$ 2.6	$ 15.0	$ 8.0	$ 11.9	$ 10.4
Revenue, remaining performance obligation, amount	$ 34.3		$ 34.3		$ 33.2
Revenue, remaining performance obligation, percentage					90.00%
Revenue, practical expedient, original expected duration			1 year		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue, Performance Obligation [Abstract]
Revenue, remaining performance obligation, percentage	91.00%		91.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period	36 months		36 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-12-31
Revenue, Performance Obligation [Abstract]
Revenue, remaining performance obligation, expected timing of satisfaction, period					1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-12-31
Revenue, Performance Obligation [Abstract]
Revenue, remaining performance obligation, expected timing of satisfaction, period					1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-12-31
Revenue, Performance Obligation [Abstract]
Revenue, remaining performance obligation, expected timing of satisfaction, period					1 year
Minimum [Member]
Revenue, Performance Obligation [Abstract]
Revenue, initial performance obligation, percentage	25.00%		25.00%		25.00%
Maximum [Member]
Revenue, Performance Obligation [Abstract]
Revenue, initial performance obligation, percentage	100.00%		100.00%			100.00%