LOANS - TDRs by portfolio (Details) - USD ($)	3 Months Ended
Mar. 31, 2020	Dec. 31, 2019
Troubled debt restructuring
Allowance for credit loss on troubled debt restructuring		$ 2,000,000.0
Allowance for loan losses on TDRs	$ 760,000
Unfunded commitments	0	0
TDRs by loan portfolio (excluding PCI loans):
TDR's Accruing	3,155,000	3,318,000
TDR's Nonaccrual	9,842,000	12,418,000
Total	12,997,000	15,736,000
Minimum \| Performing
Troubled debt restructuring
TDRs, individually evaluated for impairment, threshold	50,000
Non-PCI loans \| Commercial \| Commercial Portfolio
TDRs by loan portfolio (excluding PCI loans):
TDR's Accruing	60,000	435,000
TDR's Nonaccrual	1,181,000	369,000
Total	1,241,000	804,000
Non-PCI loans \| Commercial real estate \| Commercial Portfolio
TDRs by loan portfolio (excluding PCI loans):
TDR's Accruing	1,692,000	1,720,000
TDR's Nonaccrual	6,032,000	9,834,000
Total	7,724,000	11,554,000
Non-PCI loans \| Constructions \| Commercial Portfolio
TDRs by loan portfolio (excluding PCI loans):
TDR's Accruing	43,000	45,000
TDR's Nonaccrual	163,000	167,000
Total	206,000	212,000
Non-PCI loans \| Residential real estate \| Other loan portfolio
TDRs by loan portfolio (excluding PCI loans):
TDR's Accruing	1,328,000	1,083,000
TDR's Nonaccrual	2,414,000	1,993,000
Total	3,742,000	3,076,000
Non-PCI loans \| Consumer \| Other loan portfolio
TDRs by loan portfolio (excluding PCI loans):
TDR's Accruing	32,000	35,000
Total	32,000	35,000
Non-PCI loans \| Lease financing \| Other loan portfolio
TDRs by loan portfolio (excluding PCI loans):
TDR's Nonaccrual	52,000	55,000
Total	$ 52,000	$ 55,000

Allowance for credit loss on troubled debt restructuring.

No definition available.

Name:	msbi_AllowanceForCreditLossOnTroubledDebtRestructurings
Namespace Prefix:	msbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the individual TDR loan amount threshold for TDR loans to be individually evaluated for impairment by the entity.

+ References

No definition available.

+ Details

Name:	msbi_FinancingReceivableIndividuallyEvaluatedForImpairmentThresholdAmount
Namespace Prefix:	msbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	msbi_FinancingReceivableModificationsAbstract
Namespace Prefix:	msbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings and are accruing interest.

+ References

No definition available.

+ Details

Name:	msbi_FinancingReceivableModificationsRecordedInvestmentAccruing
Namespace Prefix:	msbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings and are not accruing interest.

+ References

No definition available.

+ Details

Name:	msbi_FinancingReceivableModificationsRecordedInvestmentNonaccrual
Namespace Prefix:	msbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable from change in methodology.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 11
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919244-210447

+ Details

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

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18845-111554

+ Details

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

- Definition

Amount of unfunded portion of contractual agreement to lend funds to borrower for financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118939409&loc=d3e18794-111554

+ Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_PerformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CollateralAxis=us-gaap_CommercialLoanMember
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_CollateralAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CollateralAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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