Unrealized Appreciation (Depreciation) and Realized Gains (Losses) on Investments - Schedule of Unrealized Appreciation (Depreciation) on Investments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Balance December 31, 2016	$ 139,700	$ 127,367
Appreciation on investments	37,797	5,012	$ (651)
Depreciation on investments	(40,067)	(9,002)	(12,543)
Gains on investments		(2,093)
Losses on investments	34,747	1,311	973
Other			0
Balance June 30, 2017	172,177	122,595	110,374
Medallion [Member]
Debt Securities, Available-for-sale [Line Items]
Balance December 31, 2016	(20,338)	(28,523)
Depreciation on investments	(38,170)	(8,670)	(12,425)
Losses on investments	34,747	825	337
Other			0
Balance June 30, 2017	(23,761)	(36,368)	(48,456)
Commercial Loans [Member]
Debt Securities, Available-for-sale [Line Items]
Balance December 31, 2016	(513)	(1,378)
Depreciation on investments	18	(332)	(118)
Losses on investments			636
Other			0
Balance June 30, 2017	(495)	(1,710)	(1,192)
Investment In Subsidiary One [Member]
Debt Securities, Available-for-sale [Line Items]
Balance December 31, 2016	158,920	152,750
Appreciation on investments	38,795	3,751	(771)
Other			0
Balance June 30, 2017	197,715	156,501	155,730
Equity Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Balance December 31, 2016	3,121	3,934
Appreciation on investments	(998)	1,261	120
Gains on investments		(2,093)
Losses on investments		486
Other			0
Balance June 30, 2017	2,123	3,588	3,708
Other than Securities Investment [Member]
Debt Securities, Available-for-sale [Line Items]
Balance December 31, 2016	(1,490)	584
Depreciation on investments	(1,915)
Other			0
Balance June 30, 2017	$ (3,405)	$ 584	584
Investment Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Other			$ 0