Summary of Significant Accounting Policies - Summary of Purchase Price Accounting (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Apr. 02, 2018	Mar. 31, 2018	Dec. 31, 2017
Investments In Loans [Line Items]
Assets		$ 1,571,407			$ 1,077,357
Assets
Cash					110,233
Other assets					58,827
Net loans		1,060,061
Other assets					58,827
Liabilities
Other liabilities					$ (3,836)
Medallion Bank [Member]
Investments In Loans [Line Items]
Assets			$ 0	$ 0
Assets
Cash		100
Other assets			130,393
Net loans	[1]		890,000
Other assets			130,393
Liabilities
Total fair value excluding goodwill and intangibles			166,743
Intangibles			28,900
Total fair value	[2]		346,446	346,446
Funds borrowed and other liabilities			(853,650)
Total fair value excluding goodwill and intangibles			166,743
Goodwill			150,803
Intangibles			28,900
Total fair value	[2]		346,446	346,446
RPAC Racing, LLC [Member]
Investments In Loans [Line Items]
Assets		$ 36,237
RPAC Racing, LLC [Member] | Operating Segments [Member]
Assets
Cash			1,647
Race cars held for sale			916
Other assets			1,902
Other assets			1,902
Liabilities
Deferred revenue			(6,531)
Notes payable	[3]		(27,220)
Other liabilities			(2,275)
Total fair value excluding goodwill and intangibles			(30,584)
Intangibles			31,779
Total fair value	[4]		1,195	1,195
Total fair value excluding goodwill and intangibles			(30,584)
Intangibles			31,779
Total fair value	[4]		1,195	$ 1,195
RPAC Racing, LLC [Member] | Operating Segments [Member] | Fixed Assets Net [Member]
Assets
Net fixed assets			774
RPAC Racing, LLC [Member] | Operating Segments [Member] | Race Cars and Parts Net [Member]
Assets
Net fixed assets			$ 203

[1] Includes $12,387 of premiums associated with the loan portfolio.

[2] Includes $26,303 of preferred stock held by the US Treasury. See Note 17 for details.

[3] Includes $20,177 due to the Company and its affiliates as of March 31, 2018.

[4] Fair value as of March 31, 2018 represents the Company's investment in RPAC Racing LLC series D units.