Summary of Significant Accounting Policies - Additional Information (Detail)			12 Months Ended
		May 26, 2017 USD ($)	Dec. 31, 2017 USD ($) Customer Loan	Dec. 31, 2016 USD ($) Loan	Dec. 31, 2015 USD ($) Loan	Jan. 01, 2018 USD ($)	Dec. 31, 2014 USD ($)
Significant Accounting Policies [Line Items]
Provision for loan losses			$ 45,614,000	$ 8,050,000	$ 8,300,000
Depreciation and amortization expense			$ 28,173,000	51,304,000	45,505,000
Number of cancelled customer relationships | Customer			1
Unpaid Principal Balance			$ 1,419,653,000	1,787,974,000
Currently available funds to repayment of financial obligations			53,380,000	110,531,000	87,581,000		$ 106,963,000
Federal deposit insurance corporation, insurance limit per institution			250,000
Restricted cash			157,914,000	190,179,000
Impairment charges on real estate assets			96,625,000
Accrued interest receivable			29,664,000	36,271,000
Earned asset management fees			915,000	1,420,000	1,892,000
Eliminated asset management fee income from securitizations			915,000	1,420,000	1,644,000
Notional amount			12,650,000	44,135,000
Variation margin payment settlement of derivative			103,000
Reversal of accrued current income tax liability			249,000
Income tax benefit			284,000	(338,000)	(314,000)
Income tax benefit, net			861,000	(2,550,000)	(2,798,000)
Intangible assets, non-cash impairment charges			7,843,000
Intangible assets, accumulated amortization			6,831,000	22,230,000
Intangible assets, amortization expenses			6,775,000	14,934,000	14,283,000
Expected amortization expense of intangible assets in 2018			1,557,000
Expected amortization expense of intangible assets in 2019			1,187,000
Expected amortization expense of intangible assets in 2020			905,000
Expected amortization expense of intangible assets in 2021			626,000
Expected amortization expense of intangible assets in 2022			425,000
Expected amortization expense of intangible assets thereafter			677,000
Goodwill			0	8,342,000
Non-cash impairment charge on goodwill			8,342,000
Derecognized of goodwill related to disposal of discontinued operations				512,000
Shareholder activism expenses			2,464,000
Employee separation expense			575,000
Subsequent Event | Revenue from Contracts with Customers
Significant Accounting Policies [Line Items]
Cumulative effect adjustment as an increase to shareholders’ equity						$ 701,000
Scenario, Adjustment
Significant Accounting Policies [Line Items]
Deferred tax liability			2,160,000
TRS Enitities
Significant Accounting Policies [Line Items]
Income tax benefit			34,000
Income tax benefit, net			12,000
London Clearing House
Significant Accounting Policies [Line Items]
Notional amount			12,650,000
Consolidation Eliminations
Significant Accounting Policies [Line Items]
Earned asset management fees				4,984,000	1,582,000
Incentive fees			350,000	629,000	154,000
IRT
Significant Accounting Policies [Line Items]
Accrued interest receivable			0	0
Earned asset management fees			0	7,092,000	4,984,000
Incentive fees			0	350,000	629,000
Property management and leasing fees			0	4,769,000	3,675,000
IRT | Consolidation Eliminations
Significant Accounting Policies [Line Items]
Earned asset management fees			0	7,092,000	4,984,000
Incentive fees			0	350,000	629,000
Property management and leasing fees			$ 0	4,769,000	3,675,000
Highland Capital Management, L. P.
Significant Accounting Policies [Line Items]
Reimbursement of certain costs incurred by related party		$ 250,000
Building and Building Improvements | Minimum
Significant Accounting Policies [Line Items]
Property, useful lives			30 years
Building and Building Improvements | Maximum
Significant Accounting Policies [Line Items]
Property, useful lives			40 years
Furniture, fixtures, and equipment | Minimum
Significant Accounting Policies [Line Items]
Property, useful lives			5 years
Furniture, fixtures, and equipment | Maximum
Significant Accounting Policies [Line Items]
Property, useful lives			10 years
Tenant escrows and borrowers' funds
Significant Accounting Policies [Line Items]
Restricted cash			$ 123,398,000	121,395,000
Securitizations
Significant Accounting Policies [Line Items]
Restricted cash			34,516,000	68,784,000
Recourse Indebtedness
Significant Accounting Policies [Line Items]
Unpaid Principal Balance			$ 313,700,000	$ 387,179,000
Recourse Indebtedness | 4.0% convertible senior notes
Significant Accounting Policies [Line Items]
Stated interest rate			4.00%	4.00%
Unpaid Principal Balance	[1]		$ 110,513,000	$ 126,098,000
Convertible senior notes redemption period			2018-10
Currently available funds to repayment of financial obligations			$ 53,380,000
Customer relationships
Significant Accounting Policies [Line Items]
Depreciation and amortization expense			$ 494,000
Number of cancelled customer relationships | Customer			7
Intangible assets, gross carrying amount			$ 0	12,276,000
Intangible assets, gross value, decreased			(12,276,000)
Intangible assets, non-cash impairment charges			4,903,000
In-place leases
Significant Accounting Policies [Line Items]
Intangible assets, gross carrying amount			12,208,000	26,122,000
Trade name
Significant Accounting Policies [Line Items]
Intangible assets, gross carrying amount			0	1,500,000
Intangible assets, non-cash impairment charges			963,000
Loan Not Collectible and Not Properly Reserved
Significant Accounting Policies [Line Items]
Provision for loan losses			1,542,000
Loans and Finance Receivables
Significant Accounting Policies [Line Items]
Accrued interest receivable			23,801,000	29,845,000	34,132,000
Impairment of loans			$ 28,904,000	$ 12,869,000	$ 0
Number of impaired loans | Loan			4	1	0
Loans and Finance Receivables | Minimum
Significant Accounting Policies [Line Items]
Stated interest rate			0.00%

[1] Our 4.0% convertible senior notes are redeemable at par, at the option of the holder, in October 2018, October 2023, and October 2028.