Secured Debt Agreements - Summary of Statistics for Asset-Specific Financings (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Participating Mortgage Loans [Line Items]
Collateral assets, Book Value	$ 826,790
Financing provided, Book Value	650,415
Collateral assets, Principal Balance	834,189
Financing provided, Principal Balance	$ 653,652
Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Collateral assets, Book Value		$ 627,839
Collateral assets, Principal Balance		$ 631,524
Count | SecurityLoan	7	5
LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	4.76%	4.44%
JP Morgan [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-01	2020-01
Collateral assets, Book Value	$ 278,963	$ 272,632
Collateral assets, Principal Balance	$ 280,415	$ 274,878
Count | SecurityLoan	1	1
JP Morgan [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	3.88%	3.88%
Citibank [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-11	2018-10
Collateral assets, Book Value	$ 201,135	$ 36,514
Collateral assets, Principal Balance	$ 201,270	$ 36,749
Count | SecurityLoan	2	1
Citibank [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	4.45%	4.42%
Deutsche Bank [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2021-08
Collateral assets, Book Value	$ 178,107
Collateral assets, Principal Balance	$ 180,740
Count | SecurityLoan	1
Deutsche Bank [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	5.17%
Bank of the Ozarks [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-02
Collateral assets, Book Value	$ 105,630
Collateral assets, Principal Balance	$ 108,423
Count | SecurityLoan	2
Bank of the Ozarks [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	6.20%
Wells Fargo [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2019-12	2019-06
Collateral assets, Book Value	$ 62,955	$ 318,693
Collateral assets, Principal Balance	$ 63,341	$ 319,897
Count | SecurityLoan	1	3
Wells Fargo [Member] | LIBOR [Member] | Collateral Assets [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	6.05%	4.92%
Asset-Specific Financings [Member]
Participating Mortgage Loans [Line Items]
Guarantee	$ 232,908	$ 102,579
Financing provided, Book Value		473,799
Financing provided, Principal Balance	$ 653,652	$ 474,655
Count | SecurityLoan	7	5
Asset-Specific Financings [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	2.57%	2.16%
Asset-Specific Financings [Member] | JP Morgan [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-01	2020-01
Guarantee	$ 116,839	$ 53,623
Financing provided, Book Value	233,541	214,391
Financing provided, Principal Balance	$ 233,679	$ 214,491
Count | SecurityLoan	1	1
Asset-Specific Financings [Member] | JP Morgan [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	1.89%	1.94%
Asset-Specific Financings [Member] | Citibank [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-11	2018-10
Guarantee	$ 39,663	$ 6,329
Financing provided, Book Value	158,605	25,293
Financing provided, Principal Balance	$ 158,652	$ 25,314
Count | SecurityLoan	2	1
Asset-Specific Financings [Member] | Citibank [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	2.48%	2.08%
Asset-Specific Financings [Member] | Deutsche Bank [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2021-08
Guarantee	$ 67,538
Financing provided, Book Value	133,599
Financing provided, Principal Balance	$ 135,075
Count | SecurityLoan	1
Asset-Specific Financings [Member] | Deutsche Bank [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	3.00%
Asset-Specific Financings [Member] | Bank of the Ozarks [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2020-02
Financing provided, Book Value	$ 80,603
Financing provided, Principal Balance	$ 81,907
Count | SecurityLoan	2
Asset-Specific Financings [Member] | Bank of the Ozarks [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	3.59%
Asset-Specific Financings [Member] | Wells Fargo [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Term	2019-12	2019-06
Guarantee	$ 8,868	$ 42,627
Financing provided, Book Value	44,067	234,115
Financing provided, Principal Balance	$ 44,339	$ 234,850
Count | SecurityLoan	1	3
Asset-Specific Financings [Member] | Wells Fargo [Member] | LIBOR [Member]
Participating Mortgage Loans [Line Items]
Weighted Average Yield/Cost Rate	3.20%	2.37%