LOANS - Summary of changes in the ACL on loans (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Impaired [Line Items]
Balance, beginning of period	$ 58,664	$ 47,093	$ 60,443	$ 28,028
Provision for credit losses on loans	0	10,970	3,950	33,149
Charge-offs	(3,660)	(5,661)	(10,271)	(22,349)
Recoveries	671	369	1,553	1,160
Balance, end of period	55,675	52,771	55,675	52,771
ASU 2016-13
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				8,546
ASU 2016-13 \| PCD (Previously PCI) loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				4,237
Commercial Loan Portfolio \| Commercial
Financing Receivable, Impaired [Line Items]
Balance, beginning of period	14,849	12,213	19,851	10,031
Provision for credit losses on loans	(75)	6,513	(2,091)	9,132
Charge-offs	(317)	(913)	(3,457)	(4,763)
Recoveries	134	47	288	88
Balance, end of period	14,591	17,860	14,591	17,860
Commercial Loan Portfolio \| Commercial \| ASU 2016-13
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				2,327
Commercial Loan Portfolio \| Commercial \| ASU 2016-13 \| PCD (Previously PCI) loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				1,045
Commercial Loan Portfolio \| Commercial real estate
Financing Receivable, Impaired [Line Items]
Balance, beginning of period	30,718	20,296	25,465	10,272
Provision for credit losses on loans	(2,105)	4,518	4,854	18,661
Charge-offs	(1,663)	(3,462)	(3,382)	(13,081)
Recoveries	3	37	16	122
Balance, end of period	26,953	21,389	26,953	21,389
Commercial Loan Portfolio \| Commercial real estate \| ASU 2016-13
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				4,104
Commercial Loan Portfolio \| Commercial real estate \| ASU 2016-13 \| PCD (Previously PCI) loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				1,311
Commercial Loan Portfolio \| Construction and land development
Financing Receivable, Impaired [Line Items]
Balance, beginning of period	1,733	1,512	1,433	290
Provision for credit losses on loans	(538)	534	(113)	233
Charge-offs	(138)	(250)	(410)	(324)
Recoveries	74	6	221	70
Balance, end of period	1,131	1,802	1,131	1,802
Commercial Loan Portfolio \| Construction and land development \| ASU 2016-13
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				724
Commercial Loan Portfolio \| Construction and land development \| ASU 2016-13 \| PCD (Previously PCI) loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				809
Residential Portfolio Segment \| Residential real estate
Financing Receivable, Impaired [Line Items]
Balance, beginning of period	3,683	4,830	3,929	2,499
Provision for credit losses on loans	(697)	(184)	(806)	226
Charge-offs	(35)	(101)	(286)	(496)
Recoveries	66	34	180	124
Balance, end of period	3,017	4,579	3,017	4,579
Residential Portfolio Segment \| Residential real estate \| ASU 2016-13
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				1,211
Residential Portfolio Segment \| Residential real estate \| ASU 2016-13 \| PCD (Previously PCI) loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				1,015
Consumer \| Consumer
Financing Receivable, Impaired [Line Items]
Balance, beginning of period	2,292	2,087	2,338	2,642
Provision for credit losses on loans	292	422	429	994
Charge-offs	(280)	(307)	(740)	(1,271)
Recoveries	93	125	370	499
Balance, end of period	2,397	2,327	2,397	2,327
Consumer \| Consumer \| ASU 2016-13
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				(594)
Consumer \| Consumer \| ASU 2016-13 \| PCD (Previously PCI) loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				57
Lease financing
Financing Receivable, Impaired [Line Items]
Balance, beginning of period	5,389	6,155	7,427	2,294
Provision for credit losses on loans	3,123	(833)	1,677	3,903
Charge-offs	(1,227)	(628)	(1,996)	(2,414)
Recoveries	301	120	478	257
Balance, end of period	$ 7,586	$ 4,814	$ 7,586	4,814
Lease financing \| ASU 2016-13
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				774
Lease financing \| ASU 2016-13 \| PCD (Previously PCI) loans
Financing Receivable, Impaired [Line Items]
Balance, beginning of period				$ 0

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

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=123577603&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: 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=124255206&loc=SL82895884-210446

Reference 4: 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=124255953&loc=SL82919249-210447

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=124255953&loc=SL82919249-210447

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (3)

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (c)

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph (a)
-Subparagraph (1)

+ Details

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

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- Definition

Amount of writeoff of financing receivable, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

+ Details

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

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(11))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

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

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
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_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=msbi_CommercialRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=msbi_ConstructionAndLandDevelopmentLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=msbi_ResidentialRealEstateLoanMember
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:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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