Summary of Significant Accounting Policies - Additional Information (Detail)		3 Months Ended			6 Months Ended		12 Months Ended
	May 26, 2017 USD ($)	Jun. 30, 2017 USD ($) Loan	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2017 USD ($) Loan	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Loan
Significant Accounting Policies [Line Items]
Investment interest expense		$ 10,546,000	$ 9,125,000		$ 20,319,000	$ 18,445,000
Federal deposit insurance corporation, insurance limit per institution		250,000			250,000
Restricted cash		193,580,000			193,580,000		$ 190,179,000
Non-cash impairment charges on real estate assets		81,444,000			88,868,000
Accrued interest receivable		32,141,000			32,141,000		36,271,000
Earned asset management fees		215,000	376,000		488,000	778,000
Intangible assets, accumulated amortization		8,776,000			8,776,000		22,230,000
Intangible assets, amortization expenses		1,913,000	$ 5,451,000		4,560,000	7,773,000
Expected amortization expense of intangible assets, remainder of 2017		1,604,000			1,604,000
Expected amortization expense of intangible assets in 2018		2,449,000			2,449,000
Expected amortization expense of intangible assets in 2019		1,904,000			1,904,000
Expected amortization expense of intangible assets in 2020		1,349,000			1,349,000
Expected amortization expense of intangible assets in 2021		1,060,000			1,060,000
Expected amortization expense of intangible assets thereafter		2,535,000			2,535,000
Notional amount		41,610,000			41,610,000		44,135,000
Variation margin payment settlement of derivative		38,000			38,000
Goodwill		0			0		8,342,000
Non-cash impairment charge on goodwill		8,342,000			8,342,000
Income tax benefit					249,000
Deferred income tax expense		22,000			22,000	$ (2,841,000)
Shareholder activism expenses		1,615,000			2,309,000
Highland Capital Management, L. P.
Significant Accounting Policies [Line Items]
Reimbursement of certain costs incurred by related party	$ 250,000
London Clearing House
Significant Accounting Policies [Line Items]
Notional amount		12,650,000			12,650,000
Customer relationships
Significant Accounting Policies [Line Items]
Intangible assets, gross carrying amount		3,134,000			3,134,000		12,276,000
Intangible assets, non-cash impairment charges		3,402,000			3,402,000
In-place leases
Significant Accounting Policies [Line Items]
Intangible assets, gross carrying amount		16,543,000			16,543,000		26,122,000
Trade name
Significant Accounting Policies [Line Items]
Intangible assets, gross carrying amount		0			0		1,500,000
Intangible assets, non-cash impairment charges		963,000			963,000
Loans and Finance Receivables
Significant Accounting Policies [Line Items]
Accrued interest receivable		$ 26,305,000			26,305,000		29,845,000
Impairment of loans					$ 24,228,000		$ 12,869,000
Number of impaired loans | Loan		3			3		1
Loans and Finance Receivables | Uncollectible and Written Off
Significant Accounting Policies [Line Items]
Accrued interest receivable written off as reduction to investment interest income		$ 3,636,000
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		112,311,000			$ 112,311,000		$ 121,395,000
Securitizations
Significant Accounting Policies [Line Items]
Restricted cash		$ 81,269,000			$ 81,269,000		$ 68,784,000
Impact on Investment Interest Expense by Increasing Investment Interest Expense and Decreasing Interest Expense
Significant Accounting Policies [Line Items]
Investment interest expense				$ 1,424,000