Fair Value of Assets and Liabilities - Additional Information (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Moratorium's expiration period	2013-07
Medallion Financing Trust I [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Appreciation in Investment in Medallion Bank	$ 39,826,000	$ 7,489,000	$ 128,918,000	$ 15,500,000