Unrealized Appreciation (Depreciation) and Realized Gains (Losses) on Investments - Schedule of Unrealized Appreciation (Depreciation) on Investments (Detail) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2017
Debt Securities, Available-for-sale [Line Items]
Beginning balance	$ 139,700	$ 110,374	$ 122,595	$ 127,367	$ 127,367
Appreciation on investments	37,797	(3,132)	(651)	5,012
Depreciation on investments	(40,067)	(6,609)	(12,543)	(9,002)
Gains on investments		(272)		(2,093)
Losses on investments	34,747	371	973	1,311
Ending balance	172,177	100,732	110,374	122,595	100,732
Medallion [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	(20,338)	(48,456)	(36,368)	(28,523)	(28,523)
Depreciation on investments	(38,170)	(6,669)	(12,425)	(8,670)
Losses on investments	34,747	311	337	825
Ending balance	(23,761)	(54,814)	(48,456)	(36,368)	(54,814)
Commercial Loans [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	(513)	(1,192)	(1,710)	(1,378)	(1,378)
Depreciation on investments	18	75	(118)	(332)
Losses on investments		60	636
Ending balance	(495)	(1,057)	(1,192)	(1,710)	(1,057)
Investment In Subsidiary One [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	158,920	155,730	156,501	152,750	152,750
Appreciation on investments	38,795	(2,771)	(771)	3,751
Ending balance	197,715	152,959	155,730	156,501	152,959
Equity Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	3,121	3,708	3,588	3,934	3,934
Appreciation on investments	(998)	(361)	120	1,261
Gains on investments		(272)		(2,093)
Losses on investments				486
Ending balance	2,123	3,075	3,708	3,588	3,075
Other than Securities Investment [Member]
Debt Securities, Available-for-sale [Line Items]
Beginning balance	(1,490)	584	584	584	584
Depreciation on investments	(1,915)	(15)
Ending balance	$ (3,405)	$ 569	$ 584	$ 584	$ 569