Debt Instrument, Issuance Price, Percentage

No definition available.

Name:	main_DebtInstrumentIssuancePricePercentage
Namespace Prefix:	main_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Increase (Decrease) In Common Stock, Dividends, Declared, Monthly

+ References

No definition available.

+ Details

Name:	main_IncreaseDecreaseInCommonStockDividendsDeclaredMonthly
Namespace Prefix:	main_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate dividends paid during the period for each share of common stock outstanding.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480070/505-10-S50-1

+ Details

Name:	us-gaap_CommonStockDividendsPerShareCashPaid
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480008/505-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480070/505-10-S50-1

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 55
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482949/835-30-55-8

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69B

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 69C
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481568/470-20-55-69C

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482900/835-30-50-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 835
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482925/835-30-45-2

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(a)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480657/210-10-S50-9

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481674/830-30-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 855
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483399/855-10-50-2

+ Details

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

SUBSEQUENT EVENTS (Details) - USD ($)		1 Months Ended	3 Months Ended					9 Months Ended		234 Months Ended
SUBSEQUENT EVENTS (Details) - USD ($)	Nov. 06, 2026	Oct. 31, 2026	Mar. 31, 2027	Dec. 31, 2026	Sep. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025	Mar. 31, 2027
Subsequent Event [Line Items]
Increase in common stock, dividends, per share, declared, monthly (as a percent)						3.80%
Dividends paid, per share (in dollars per share)					$ 1.095		$ 1.065	$ 3.255	$ 3.165
O 2026 Q4 Supplemental Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)	$ 0.30
O 2026 M10 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)	0.265
O 2026 M11 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)	0.265
O 2026 M12 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)	0.265
O 2026 Q4 Dividends \| Forecast
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)				$ 0.795
O 2026 Q4 Including Supplemental Dividends \| Forecast
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)				$ 1.095
O 2027 M1 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)	0.27
O 2027 M2 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)	0.27
O 2027 M3 Dividends \| Subsequent Event
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)	$ 0.27
O 2027 Q1 Dividends \| Forecast
Subsequent Event [Line Items]
Common stock, dividend, per share, declared (in dollars per share)			$ 0.81
O 2027 Q1 Life-To-Date Dividends \| Forecast
Subsequent Event [Line Items]
Dividends paid, per share (in dollars per share)										$ 52.315
January 2030 Notes \| Subsequent Event \| Unsecured Notes
Subsequent Event [Line Items]
Face amount		$ 500,000,000.0
Debt instrument, interest rate, stated (as a percent)		6.55%
Debt instrument, issuance price (as a percent)		99.258%