This line item represents the interest received on nonaccrual loans.

No definition available.

Name:	msl_InterestReceivedOnNonaccrualLoans
Namespace Prefix:	msl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This line item represents the loans exposure in the oil and gas industry.

+ References

No definition available.

+ Details

Name:	msl_LoansExposureInOilAndGasIndustry
Namespace Prefix:	msl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This line item represents the loans with exposure in the commercial real estate section.

+ References

No definition available.

+ Details

Name:	msl_LoansWithExposureInCommercialRealEstate
Namespace Prefix:	msl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This line item represents the percentage of nonaccrual status of loans specified as percentage of total CRE loans.

+ References

No definition available.

+ Details

Name:	msl_NonaccrualStatusofLoansSpecifiedasaPercentageofTotalCRELoans
Namespace Prefix:	msl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This line item represents the number of industry segment concentration above threshold limit of the loan portfolio.

+ References

No definition available.

+ Details

Name:	msl_NumberOfIndustrySegmentConcentrationAboveThresholdLimit
Namespace Prefix:	msl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Total CRE loans on non-accrual

+ References

No definition available.

+ Details

Name:	msl_TotalCREloansonnonaccrual
Namespace Prefix:	msl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

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

- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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

- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18794-111554

+ Details

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

- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_LoansAndFinanceReceivablesMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
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Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=msl_OwnerOccupiedCommercialPropertyMember
Namespace Prefix:
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Loans - Narrative (Details)	3 Months Ended		9 Months Ended
Loans - Narrative (Details)	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 contract	Sep. 30, 2018 USD ($) industry_concentration contract	Sep. 30, 2017 USD ($) contract	Dec. 31, 2017 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Number of industry segment concentration above threshold limit \| industry_concentration			1
Concentration risk (as a percent)			10.00%
Loans exposure in oil and gas industry	$ 128,100,000		$ 128,100,000
Loans with exposure in commercial real estate	484,900,000		484,900,000
Loans on nonaccrual status	$ 51,476,000		$ 51,476,000		$ 49,278,000
Nonaccrual status of loans specified as percentage of total CRE loans	5.00%		5.00%
Interest lost on nonaccrual loans			$ 5,000,000	$ 2,600,000
Interest received on nonaccrual loans			253,000	$ 201,000
Commitments to lend	$ 0		$ 0
Defaults on loans \| contract	0	0	1	6
Pre-modification balance			$ 18,000	$ 2,000,000
Real estate – commercial
Financing Receivable, Recorded Investment [Line Items]
Total CRE loans on non-accrual	$ 24,100,000		24,100,000
Loans on nonaccrual status	$ 23,468,000		$ 23,468,000		$ 11,128,000
Loans and Finance Receivables \| Customer Concentration Risk \| Manufacturing Industry
Financing Receivable, Recorded Investment [Line Items]
Concentration risk (as a percent)			13.30%
Loans and Finance Receivables \| Customer Concentration Risk \| Real estate – commercial
Financing Receivable, Recorded Investment [Line Items]
Concentration risk (as a percent)			45.50%
Loans and Finance Receivables \| Customer Concentration Risk \| Real estate – commercial \| Owner Occupied Commercial Property
Financing Receivable, Recorded Investment [Line Items]
Concentration risk (as a percent)			52.00%