Allowance for Loan and Lease Losses (Recorded Investment) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Impaired Loans and Leases
Recorded investment	$ 58,712	$ 63,765
Unpaid principal balance	58,757	63,839
Related allowance	1,789	3,081
Average recorded investment	62,429	62,035	$ 72,845
Interest income recognized, Total	1,496	1,329	1,404
Recorded investment in loans and leases by portfolio segment
Total, Allowance	61,082	58,692
Total Loans and Leases	6,737,816	6,303,516
Originated
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	34,675	39,485
Recorded investment, loans with related allowance recorded	7,272	9,285
Recorded investment	41,947	48,770
Unpaid principal balance with no related allowance recorded	34,648	39,576
Unpaid principal balance with related allowance recorded	7,343	9,268
Unpaid principal balance	41,991	48,844
Related allowance	1,749	3,055
Recorded investment, nonaccrual loans	18,500	22,700
Individually evaluated for impairment, loans and leases	27,300	41,200
Collectively evaluated for impairment, loans and leases	14,600	7,600
Average recorded investment with no related allowance recorded	37,569	35,799	39,461
Average recorded investment with related allowance recorded	8,340	9,960	21,653
Average recorded investment	45,909	45,759	61,114
Interest income recognized with no related allowance recorded	1,161	1,134	1,082
Interest income recognized with related allowance recorded	111	107	163
Interest income recognized, Total	1,272	1,241	1,245
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	1,749	3,055
Collectively evaluated for impairment, allowance	57,118	53,823
Total, Allowance	58,867	56,878
Individually evaluated for impairment, loans and leases	27,301	41,239
Collectively evaluated for impairment, loans and leases	6,422,386	5,867,870
Total Loans and Leases	6,449,687	5,909,109
Acquired
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	16,318	14,841
Recorded investment, loans with related allowance recorded	447	154
Recorded investment	16,765	14,995
Unpaid principal balance with no related allowance recorded	16,319	14,841
Unpaid principal balance with related allowance recorded	447	154
Unpaid principal balance	16,766	14,995
Related allowance	40	26
Recorded investment, nonaccrual loans	900	1,300
Individually evaluated for impairment, loans and leases	5,200	2,500
Collectively evaluated for impairment, loans and leases	11,600	12,500
Average recorded investment with no related allowance recorded	16,218	16,141	11,580
Average recorded investment with related allowance recorded	302	135	151
Average recorded investment	16,520	16,276	11,731
Interest income recognized with no related allowance recorded	213	84	155
Interest income recognized with related allowance recorded	11	4	4
Interest income recognized, Total	224	88	159
Recorded investment in loans and leases by portfolio segment
Total, Allowance	2,215	1,814
Total Loans and Leases	288,129	394,407
Financial Asset Acquired and No Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	40	26
Collectively evaluated for impairment, allowance	273	135
Individually evaluated for impairment, loans and leases	5,180	2,476
Collectively evaluated for impairment, loans and leases	205,688	287,407
Financial Asset Acquired with Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Total, Allowance	1,902	1,653
Total Loans and Leases	77,261	104,524
Commercial real estate loans
Recorded investment in loans and leases by portfolio segment
Total, Allowance	30,285	28,187
Total Loans and Leases	3,669,222	3,351,736
Commercial real estate loans \| Originated
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	3,899	5,569
Recorded investment, loans with related allowance recorded	68	396
Unpaid principal balance with no related allowance recorded	3,892	5,545
Unpaid principal balance with related allowance recorded	68	396
Related allowance	7	5
Average recorded investment with no related allowance recorded	5,148	6,484	10,181
Average recorded investment with related allowance recorded	269	99	3,271
Interest income recognized with no related allowance recorded	110	87	277
Interest income recognized with related allowance recorded	3	0	162
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	7	5
Collectively evaluated for impairment, allowance	28,415	26,617
Total, Allowance	28,422	26,622
Individually evaluated for impairment, loans and leases	3,956	5,610
Collectively evaluated for impairment, loans and leases	3,531,578	3,154,245
Total Loans and Leases	3,535,534	3,159,855
Commercial real estate loans \| Acquired
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	12,365	9,538
Unpaid principal balance with no related allowance recorded	12,366	9,538
Average recorded investment with no related allowance recorded	11,409	9,868	4,005
Interest income recognized with no related allowance recorded	163	7	55
Recorded investment in loans and leases by portfolio segment
Total, Allowance	1,863	1,565
Total Loans and Leases	133,688	191,881
Commercial real estate loans \| Financial Asset Acquired and No Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	0	0
Collectively evaluated for impairment, allowance	65	32
Individually evaluated for impairment, loans and leases	2,942	0
Collectively evaluated for impairment, loans and leases	79,465	121,119
Commercial real estate loans \| Financial Asset Acquired with Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Total, Allowance	1,798	1,533
Total Loans and Leases	51,281	70,762
Commercial
Recorded investment in loans and leases by portfolio segment
Total, Allowance	24,826	25,283
Total Loans and Leases	1,838,748	1,768,958
Commercial \| Originated
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	28,539	30,927
Recorded investment, loans with related allowance recorded	5,980	8,224
Unpaid principal balance with no related allowance recorded	28,533	31,053
Unpaid principal balance with related allowance recorded	6,055	8,208
Related allowance	1,672	2,961
Average recorded investment with no related allowance recorded	29,759	26,514	24,950
Average recorded investment with related allowance recorded	7,125	9,026	18,382
Interest income recognized with no related allowance recorded	1,009	993	747
Interest income recognized with related allowance recorded	76	96	1
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	1,672	2,961
Collectively evaluated for impairment, allowance	22,853	22,131
Total, Allowance	24,525	25,092
Individually evaluated for impairment, loans and leases	20,019	32,127
Collectively evaluated for impairment, loans and leases	1,800,691	1,709,794
Total Loans and Leases	1,820,710	1,741,921
Commercial \| Acquired
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	437	531
Unpaid principal balance with no related allowance recorded	437	531
Average recorded investment with no related allowance recorded	511	1,212	2,280
Interest income recognized with no related allowance recorded	11	16	31
Recorded investment in loans and leases by portfolio segment
Total, Allowance	301	191
Total Loans and Leases	18,038	27,037
Commercial \| Financial Asset Acquired and No Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	0	0
Collectively evaluated for impairment, allowance	197	83
Individually evaluated for impairment, loans and leases	397	404
Collectively evaluated for impairment, loans and leases	15,465	24,094
Commercial \| Financial Asset Acquired with Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Total, Allowance	104	108
Total Loans and Leases	2,176	2,539
Consumer loans
Recorded investment in loans and leases by portfolio segment
Total, Allowance	5,971	5,222
Total Loans and Leases	1,229,846	1,182,822
Consumer loans \| Originated
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	2,237	2,989
Recorded investment, loans with related allowance recorded	1,224	665
Unpaid principal balance with no related allowance recorded	2,223	2,978
Unpaid principal balance with related allowance recorded	1,220	664
Related allowance	70	89
Average recorded investment with no related allowance recorded	2,662	2,801	4,330
Average recorded investment with related allowance recorded	946	835	0
Interest income recognized with no related allowance recorded	42	54	58
Interest income recognized with related allowance recorded	32	11	0
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	70	89
Collectively evaluated for impairment, allowance	5,850	5,075
Total, Allowance	5,920	5,164
Individually evaluated for impairment, loans and leases	3,326	3,502
Collectively evaluated for impairment, loans and leases	1,090,117	1,003,831
Total Loans and Leases	1,093,443	1,007,333
Consumer loans \| Acquired
Impaired Loans and Leases
Recorded investment, loans with no related allowance recorded	3,516	4,772
Recorded investment, loans with related allowance recorded	447	154
Unpaid principal balance with no related allowance recorded	3,516	4,772
Unpaid principal balance with related allowance recorded	447	154
Related allowance	40	26
Average recorded investment with no related allowance recorded	4,298	5,061	5,295
Average recorded investment with related allowance recorded	302	135	151
Interest income recognized with no related allowance recorded	39	61	69
Interest income recognized with related allowance recorded	11	4	$ 4
Recorded investment in loans and leases by portfolio segment
Total, Allowance	51	58
Total Loans and Leases	136,403	175,489
Consumer loans \| Financial Asset Acquired and No Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Individually evaluated for impairment, allowance	40	26
Collectively evaluated for impairment, allowance	11	20
Individually evaluated for impairment, loans and leases	1,841	2,072
Collectively evaluated for impairment, loans and leases	110,758	142,194
Consumer loans \| Financial Asset Acquired with Credit Deterioration
Recorded investment in loans and leases by portfolio segment
Total, Allowance	0	12
Total Loans and Leases	$ 23,804	$ 31,223

Impaired Financing Receivable, Collectively Evaluated for Impairment

No definition available.

Name:	brkl_ImpairedFinancingReceivableCollectivelyEvaluatedforImpairment
Namespace Prefix:	brkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Impaired Financing Receivable, Individually Evaluated for Impairment

+ References

No definition available.

+ Details

Name:	brkl_ImpairedFinancingReceivableIndividuallyEvaluatedforImpairment
Namespace Prefix:	brkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Impaired Financing Receivable, Recorded Investment, Nonaccrual

+ References

No definition available.

+ Details

Name:	brkl_ImpairedFinancingReceivableRecordedInvestmentNonaccrual
Namespace Prefix:	brkl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	brkl_RecordedInvestmentInLoansAndLeasesByPortfolioSegmentAbstract
Namespace Prefix:	brkl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118955202&loc=SL82895884-210446

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

+ Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5258-111524

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953550-111524

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5258-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

+ Details

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

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetOriginatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=brkl_AcquiredFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredAndNoCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredWithCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: