Loans Considered to be Impaired (Detail) - USD ($) $ in Thousands	Mar. 31, 2019		Dec. 31, 2018		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	$ 39,349		$ 40,782
Impaired loans with reserves	76,269		85,863
Total Impaired Loans	115,618	[1]	126,645	[2]
Allowance for loan losses	20,727		22,336		$ 13,732	$ 14,883
Commercial Mortgage Loans
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	7,148		8,579
Impaired loans with reserves	63,904		73,438
Total Impaired Loans	71,052	[1]	82,017	[2]
Allowance for loan losses	10,401		11,912		8,259	9,019
Mezzanine Loans
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	12,869		12,869
Impaired loans with reserves	8,245		8,245
Total Impaired Loans	21,114	[1]	21,114	[2]
Allowance for loan losses	6,496		6,496		5,231	5,622
Preferred Equity Interests
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	19,332		19,334
Impaired loans with reserves	4,120		4,180
Total Impaired Loans	23,452	[1]	23,514	[2]
Allowance for loan losses	$ 3,830		$ 3,928		$ 242	$ 242

[1] As of March 31, 2019, there was no unpaid principal relating to previously identified TDRs that are on accrual status.

[2] As of December 31, 2018, there was no unpaid principal relating to previously identified TDRs that are on accrual status.