Fair Value of Assets and Liabilities - Schedule of Changes in Fair Value of the Company's Level 3 Assets and Liabilities (Detail) - USD ($)		3 Months Ended						6 Months Ended		9 Months Ended
		Sep. 30, 2018		Mar. 31, 2018		Sep. 30, 2017		Sep. 30, 2018		Sep. 30, 2017
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Beginning balance				$ 7,450,000
Ending balance						$ 9,510,000				$ 9,510,000
Fair Value, Measurements, Nonrecurring [Member] | Taxi Medallion Loan Trust III [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Beginning balance				208,279,000		233,415,000		$ 161,155,000		266,816,000
Gains (losses) included in earnings				(38,190,000)		(6,690,000)				(27,837,000)
Purchases, investments, and issuances				7,000		1,475,000				1,795,000
Sales, maturities, settlements, and distributions				(8,941,000)		(3,620,000)				(16,194,000)
Ending balance				161,155,000		224,580,000				224,580,000
Amounts related to held assets				(38,190,000)	[1]	(6,669,000)	[2]			(27,764,000)	[2]
Fair Value, Measurements, Nonrecurring [Member] | Commercial Loan And Lease [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Beginning balance				90,188,000		78,092,000		93,620,000		83,634,000
Gains (losses) included in earnings				(8,000)		(73,000)				(476,000)
Purchases, investments, and issuances				7,252,000		6,007,000				13,823,000
Sales, maturities, settlements, and distributions				(3,812,000)		(1,266,000)				(14,221,000)
Ending balance				93,620,000		82,760,000				82,760,000
Amounts related to held assets				(10,000)	[1]	75,000	[2]			(375,000)	[2]
Fair Value, Measurements, Nonrecurring [Member] | Investment [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Beginning balance				302,147,000		301,819,000		331,169,000		293,360,000
Gains (losses) included in earnings				29,143,000		3,291,000				12,345,000
Purchases, investments, and issuances				462,000		250,000				652,000
Sales, maturities, settlements, and distributions				(583,000)		(1,499,000)				(2,496,000)
Ending balance				331,169,000		303,861,000				303,861,000
Amounts related to held assets				29,143,000	[1]	3,291,000	[2]			12,345,000	[2]
Fair Value, Measurements, Nonrecurring [Member] | Equity Investment [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Beginning balance		$ 10,773,000		9,521,000		10,316,000		9,458,000		8,407,000
Gains (losses) included in earnings		(400,000)		(993,000)		(325,000)		(774,000)		3,830,000
Purchases, investments, and issuances		631,000		935,000		300,000		1,160,000		1,156,000
Sales, maturities, settlements, and distributions		(252,000)		(5,000)		(307,000)		(469,000)		(3,409,000)
Transfers in	[3]							1,377,000
Ending balance		10,752,000		9,458,000		9,984,000		10,752,000		9,984,000
Amounts related to held assets		$ (400,000)	[4]	(993,000)	[1]	(325,000)	[2]	(774,000)	[4]	1,056,000	[2]
Fair Value, Measurements, Nonrecurring [Member] | Other than Securities Investment [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Beginning balance				7,450,000		9,510,000		5,535,000		9,510,000
Gains (losses) included in earnings				(1,915,000)
Ending balance				5,535,000		9,510,000				9,510,000
Amounts related to held assets	[1]			(1,915,000)
Fair Value, Measurements, Nonrecurring [Member] | Other Asset [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Beginning balance				339,000		354,000		$ 339,000		354,000
Gains (losses) included in earnings						(15,000)				(15,000)
Ending balance				$ 339,000		339,000				339,000
Amounts related to held assets	[2]					$ (15,000)				$ (15,000)

[1] Total realized and unrealized gains (losses) included in income for the period which relate to assets held as of March 31, 2018

[2] Total realized and unrealized gains (losses) included in income for the period which relate to assets held as of September 30, 2017.

[3] Represents the removal of RPAC investments eliminated in consolidation as well as the transfer of LAX from controlled subsidiaries during the 2018 second quarter.

[4] Total realized and unrealized gains (losses) included in income for the period which relate to assets held as of September 30, 2018.