Summary of Total Recourse and Non-Recourse Indebtedness (Detail) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019		Dec. 31, 2018
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 591,296		$ 642,559
Unamortized Discount/Premium and Deferred Financing Costs		(16,033)		(16,643)
Indebtedness		$ 575,263		$ 625,916
Weighted-Average Interest Rate		4.80%		4.60%
Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 165,451		$ 165,451
Unamortized Discount/Premium and Deferred Financing Costs		(14,562)		(14,144)
Indebtedness		$ 150,889		$ 151,307
Weighted-Average Interest Rate		7.00%		6.90%
Non Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 425,845		$ 477,108
Unamortized Discount/Premium and Deferred Financing Costs		(1,471)		(2,499)
Indebtedness		$ 424,374		$ 474,609
Weighted-Average Interest Rate		4.00%		3.90%
7.625% Senior Notes due 2024 | Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 56,324		$ 56,324
Unamortized Discount/Premium and Deferred Financing Costs		(1,166)		(1,224)
Indebtedness		$ 55,158		$ 55,100
Weighted-Average Interest Rate		7.60%		7.60%
Contractual Maturity, month and year		2024-04		2024-04
7.125% Senior Notes due 2019 | Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 65,356		$ 65,356
Unamortized Discount/Premium and Deferred Financing Costs		(225)		(359)
Indebtedness		$ 65,131		$ 64,997
Weighted-Average Interest Rate		7.10%		7.10%
Contractual Maturity, month and year		2019-08		2019-08
Junior Subordinated Notes, at Fair Value | Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance	[1]	$ 18,671		$ 18,671
Unamortized Discount/Premium and Deferred Financing Costs	[1]	(13,171)		(12,561)
Indebtedness	[1]	$ 5,500		$ 6,110
Weighted-Average Interest Rate	[1]	6.80%		6.40%
Contractual Maturity, month and year	[1]	2035-03		2035-03
Junior subordinated notes, at amortized cost | Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 25,100		$ 25,100
Indebtedness		$ 25,100		$ 25,100
Weighted-Average Interest Rate		5.30%		5.00%
Contractual Maturity, month and year		2037-04		2037-04
CDO notes payable, at amortized cost | Non Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 102,918	[2],[3]	$ 116,102	[4],[5]
Indebtedness		$ 102,918	[2],[3]	$ 116,102	[4],[5]
Weighted-Average Interest Rate		3.20%	[2],[3]	3.10%	[4],[5]
Contractual Maturity, month and year		2046-11	[2],[3]	2046-11	[4],[5]
Loan Payable On Real Estate | Non Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance		$ 40,534		$ 40,724
Unamortized Discount/Premium and Deferred Financing Costs		(204)		(225)
Indebtedness		$ 40,330		$ 40,499
Weighted-Average Interest Rate		4.70%		4.70%
Loan Payable On Real Estate | Non Recourse Indebtedness | Minimum
Debt Instrument [Line Items]
Contractual Maturity, month and year		2021-10		2021-10
Loan Payable On Real Estate | Non Recourse Indebtedness | Maximum
Debt Instrument [Line Items]
Contractual Maturity, month and year		2021-12		2021-12
CMBS securitizations | Non Recourse Indebtedness
Debt Instrument [Line Items]
Unpaid Principal Balance	[7]	$ 282,393	[6]	$ 320,282	[8]
Unamortized Discount/Premium and Deferred Financing Costs	[7]	(1,267)	[6]	(2,274)	[8]
Indebtedness	[7]	$ 281,126	[6]	$ 318,008	[8]
Weighted-Average Interest Rate	[7]	4.20%	[6]	4.00%	[8]
CMBS securitizations | Non Recourse Indebtedness | Minimum
Debt Instrument [Line Items]
Contractual Maturity, month and year	[7]	2037-06	[6]	2037-06	[8]
CMBS securitizations | Non Recourse Indebtedness | Maximum
Debt Instrument [Line Items]
Contractual Maturity, month and year	[7]	2037-12	[6]	2037-12	[8]

[1] Relates to liabilities which we elected to record at fair value under FASB ASC Topic 825.

[2] Collateralized by $150,845 principal amount of commercial mortgage loans, mezzanine loans, other loans and preferred equity interests, $78,979 of which is eliminated in consolidation. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[3] Excludes CDO notes payable purchased by us which are eliminated in consolidation.

[4] Collateralized by $170,124 principal amount of commercial mortgage loans, mezzanine loans, other loans and preferred equity interests, $94,191 of which is eliminated in consolidation. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[5] Excludes CDO notes payable purchased by us which are eliminated in consolidation.

[6] Collateralized by $355,699 principal amount of commercial mortgage loans and participation interests in commercial mortgage loans, $8,892 of which is eliminated in consolidation. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.

[7] Excludes CMBS securitization notes purchased by us which are eliminated in consolidation.

[8] Collateralized by $409,218 principal amount of commercial mortgage loans and participation interests in commercial mortgage loans. These obligations were issued by separate legal entities and consequently the assets of the special purpose entities that collateralize these obligations are not available to our creditors.