Credit Quality of Loans and Allowance for Loan Losses - Narrative (Details)	3 Months Ended
Mar. 31, 2018 USD ($) industry_concentration	Dec. 31, 2017 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Number of industry segment concentration above threshold limit \| industry_concentration	1
Concentration Risk, Percentage	10.00%
Loans exposure in oil and gas industry	$ 172,800,000
Exposure in the oil and gas industry specified as percentage of total loans	15.20%
Loans with exposure in commercial real estate	$ 502,500,000
Loans on nonaccrual status	$ 82,275,000	$ 49,278,000
Nonaccrual status of loans specified as percentage of total CRE loans	5.20%
Loans and Leases Receivable, Impaired, Commitment to Lend	$ 0
Real estate – commercial
Financing Receivable, Recorded Investment [Line Items]
Percentage of CRE loans secured by owner-occupied commercial properties	54.00%
Total CRE loans on non-accrual	$ 26,200,000
Loans on nonaccrual status	$ 26,006,000	$ 11,128,000

This line item represents the exposure in the oil and gas industry specified as a percentage of total loans.

No definition available.

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

- 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

This line item represents the percentage of CRE loans secured by owner occupied commercial properties.

+ References

No definition available.

+ Details

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

- 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13537-108611

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5111-111524

+ Details

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

+ Details

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

- Details

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