Fair Value of Assets and Liabilities - Summary of Valuation Techniques and Significant Unobservable Inputs Used in Recurring Level 3 Fair Value Measurements of Assets and Liabilities (Detail) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment		$ 43,478
Medallion Loans [Member] | Level 3 [Member] | Precedent Market Transactions [Member] | Measurement Input Adequacy of Collateral Loan to Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans		208,279
Commercial Loans [Member] | Level 3 [Member] | Borrower Financial Analysis [Member] | Measurement Input Financial Condition and Operational Performance [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans		$ 90,188
Investment in Medallion Bank [Member] | Cash Flow Analysis [Member] | Measurement Input, Conversion Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment, measurement input		0.1750
Investment in Medallion Bank [Member] | Level 3 [Member] | Valuation Technique Precedent Merger and Acquisition Transactions [Member] | Price Book Value Multiples [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment		$ 290,548
Investment in Other Controlled Subsidiaries [Member] | Level 3 [Member] | Investee Financial Analysis [Member] | Financial Condition and Operating Performance of the Investee [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment		4,623
Investment in Other Controlled Subsidiaries [Member] | Level 3 [Member] | Investee Book Value Adjusted for Asset Appreciation [Member] | Third Party Valuation or Offer to Purchase Asset [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment		3,878
Investment in Other Controlled Subsidiaries [Member] | Level 3 [Member] | Investee Book Value Adjusted for Market Appreciation [Member] | Third Party Offer to Purchase Investment [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment		3,001
Investment in Other Controlled Subsidiaries [Member] | Level 3 [Member] | Investee Book Value and Equity Pickup [Member] | Financial Condition and Operating Performance of the Investee [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment		$ 97
Equity Investments [Member] | Precedent Market Transactions [Member] | Equity Method Offering Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value | $ / shares	$ 8.73	$ 8.73
Equity Investments [Member] | Investee Financial Analysis [Member] | Measurement Input, Credit Spread [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity investment, input		0.12
Equity Investments [Member] | Level 3 [Member] | Precedent Market Transactions [Member] | Equity Method Offering Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Investments	$ 1,455	$ 1,455
Equity Investments [Member] | Level 3 [Member] | Investee Financial Analysis [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Investments		5,417
Equity Investments [Member] | Level 3 [Member] | Investee Financial Analysis [Member] | Measurement Input Financial Condition and Operational Performance [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Investments	6,306
Equity Investments [Member] | Level 3 [Member] | Investee Financial Analysis [Member] | Measurement Input, Share Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Investments		2,193
Equity Investments [Member] | Level 3 [Member] | Investee Book Value Adjusted for Market Appreciation [Member] | Financial Condition and Operating Performance of the Investee [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Investments	2,556
Equity Investments [Member] | Level 3 [Member] | Investee Book Value [Member] | Measurement Input, Transfer prices of Chicago medallions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Investments		$ 456
Equity Investments [Member] | Level 3 [Member] | Investee Book Value [Member] | Equity Method Valuation Indicated By Investee Filings [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Investments	456
Other than Securities Investment [Member] | Cash Flow Analysis [Member] | Discount Rate in Cash Flow Analysis [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments other than securities		0.06
Other than Securities Investment [Member] | Level 3 [Member] | Cash Flow Analysis [Member] | Discount Rate in Cash Flow Analysis [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments other than securities		$ 7,450
Other Debt Obligations [Member] | Borrower Collateral Analysis [Member] | Measurement Input, Adequacy of Collateral [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other assets, input		0.00
Other Debt Obligations [Member] | Level 3 [Member] | Borrower Collateral Analysis [Member] | Measurement Input, Adequacy of Collateral [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other assets		$ 339
Minimum [Member] | Medallion Loans [Member] | Precedent Market Transactions [Member] | Measurement Input Adequacy of Collateral Loan to Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans, measurement input		0.01
Minimum [Member] | Commercial Loans [Member] | Borrower Financial Analysis [Member] | Measurement Input Financial Condition and Operational Performance [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans, measurement input		0.0200
Minimum [Member] | Investment in Medallion Bank [Member] | Valuation Technique Precedent Merger and Acquisition Transactions [Member] | Price Book Value Multiples [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment, measurement input		2.1
Minimum [Member] | Investment in Medallion Bank [Member] | Valuation Technique Precedent Merger and Acquisition Transactions [Member] | Price Earnings Multiple [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment, measurement input		8.7
Minimum [Member] | Investment in Medallion Bank [Member] | Cash Flow Analysis [Member] | Terminal Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		$ 470,964
Minimum [Member] | Investment in Other Controlled Subsidiaries [Member] | Investee Financial Analysis [Member] | Financial Condition and Operating Performance Enterprise Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		37,500
Minimum [Member] | Investment in Other Controlled Subsidiaries [Member] | Investee Financial Analysis [Member] | Financial Condition and Operating Performance Equity Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		2,000
Minimum [Member] | Equity Investments [Member] | Investee Financial Analysis [Member] | Measurement Input, Share Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		$ 2,000
Minimum [Member] | Equity Investments [Member] | Precedent Arms Length Offer [Member] | Financial Condition and Operating Performance Enterprise Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value	$ 6,018
Equity investment, input	0.94
Maximum [Member] | Medallion Loans [Member] | Precedent Market Transactions [Member] | Measurement Input Adequacy of Collateral Loan to Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans, measurement input		4.20
Maximum [Member] | Commercial Loans [Member] | Borrower Financial Analysis [Member] | Measurement Input Financial Condition and Operational Performance [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans, measurement input		0.1900
Maximum [Member] | Investment in Medallion Bank [Member] | Valuation Technique Precedent Merger and Acquisition Transactions [Member] | Price Book Value Multiples [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment, measurement input		2.5
Maximum [Member] | Investment in Medallion Bank [Member] | Valuation Technique Precedent Merger and Acquisition Transactions [Member] | Price Earnings Multiple [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investment, measurement input		10.6
Maximum [Member] | Investment in Medallion Bank [Member] | Cash Flow Analysis [Member] | Terminal Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		$ 623,007
Maximum [Member] | Investment in Other Controlled Subsidiaries [Member] | Investee Financial Analysis [Member] | Financial Condition and Operating Performance Enterprise Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		41,500
Maximum [Member] | Investment in Other Controlled Subsidiaries [Member] | Investee Financial Analysis [Member] | Financial Condition and Operating Performance Equity Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		5,000
Maximum [Member] | Equity Investments [Member] | Investee Financial Analysis [Member] | Measurement Input, Share Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value		$ 5,000
Maximum [Member] | Equity Investments [Member] | Precedent Arms Length Offer [Member] | Financial Condition and Operating Performance Enterprise Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value	$ 7,218
Equity investment, input	4.42
Weighted Average [Member] | Medallion Loans [Member] | Precedent Market Transactions [Member] | Measurement Input Adequacy of Collateral Loan to Value [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans, measurement input		1.31
Weighted Average [Member] | Commercial Loans [Member] | Borrower Financial Analysis [Member] | Measurement Input Financial Condition and Operational Performance [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans, measurement input		0.1202