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Unrealized Appreciation (Depreciation) and Realized Gains (Losses) on Investments - Schedule of Unrealized Appreciation (Depreciation) on Investments (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Beginning balance	$ 139,700
Appreciation on investments	37,797
Depreciation on investments	(40,067)
Losses on investments	34,747
Ending balance	172,177
Medallion [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	(20,338)
Depreciation on investments	(38,170)
Losses on investments	34,747
Ending balance	(23,761)
Commercial Loans [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	(513)
Depreciation on investments	18
Ending balance	(495)
Investments in Subsidiaries [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	158,920
Appreciation on investments	38,795
Ending balance	197,715
Equity Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	3,121
Appreciation on investments	(998)
Ending balance	2,123
Investments Other than Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	(1,490)
Depreciation on investments	(1,915)
Ending balance	$ (3,405)

X

- Definition

Appreciation on investment.

+ References

No definition available.

+ Details

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Namespace Prefix:	mfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Depreciation on investments

+ References

No definition available.

+ Details

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Namespace Prefix:	mfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Reverse in unrealized loss on investments.

+ References

No definition available.

+ Details

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Namespace Prefix:	mfin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

This element represents the net excess or deficiency of the fair value of an investment (security, contract) over or under its cost (face amount, notional amount), respectively, which excess value or deficiency has not been recognized in earnings of the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=120401592&loc=SL6090603-123010

+ Details

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X

- 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

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