Funds Borrowed - Summary of Key Attributes of Various Borrowing Arrangements with Lenders (Parenthetical) (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($)
Notes Payable [Line Items]
Guaranteed amount	$ 12,708,000
Minimum [Member]
Notes Payable [Line Items]
Minimum monthly payments of pledged loan	0
Maximum [Member]
Notes Payable [Line Items]
Minimum monthly payments of pledged loan	$ 75,000
Parent Company [Member] | Notes Payable to Banks [Member]
Notes Payable [Line Items]
Description of variable rate basis	30 day LIBOR was 2.09%, 360 day LIBOR was 2.76%
Parent Company [Member] | Notes Payable to Banks [Member] | Prime Rate [Member]
Notes Payable [Line Items]
Debt Instrument interest rate, stated percentage	5.00%
Parent Company [Member] | Notes Payable to Banks [Member] | 30 Day LIBOR [Member]
Notes Payable [Line Items]
Debt Instrument interest rate, stated percentage	2.09%
Parent Company [Member] | Notes Payable to Banks [Member] | 360 Day LIBOR [Member]
Notes Payable [Line Items]
Debt Instrument interest rate, stated percentage	2.76%
Medallion Chicago [Member] | Notes Payable to Banks [Member]
Notes Payable [Line Items]
Debt Instrument interest rate, stated percentage	3.75%
Description of variable rate basis	One note has an interest rate of Prime, one note has an interest rate of Prime plus 0.50%, one note has a fixed interest rate of 3.75%, one note has an interest rate of LIBOR plus 3.75%, and the other interest rates on these borrowings are LIBOR plus 2%.
Medallion Chicago [Member] | Notes Payable to Banks [Member] | Prime Rate Plus [Member]
Notes Payable [Line Items]
Basis spread on variable rate	0.50%
Medallion Chicago [Member] | Notes Payable to Banks [Member] | Fixed Interest Rate [Member]
Notes Payable [Line Items]
Basis spread on variable rate	3.75%
Medallion Chicago [Member] | Notes Payable to Banks [Member] | LIBOR Rate [Member]
Notes Payable [Line Items]
Basis spread on variable rate	2.00%