Loans Considered to be Impaired (Detail) - USD ($) $ in Thousands	Jun. 30, 2019		Mar. 31, 2019	Dec. 31, 2018		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	$ 37,298			$ 40,782
Impaired loans with reserves	68,754			85,863
Total Impaired Loans	106,052	[1]		126,645	[2]
Allowance for loan losses	20,145		$ 20,727	22,336		$ 16,699	$ 13,732	$ 12,354
Commercial Mortgage Loans
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	5,099			8,579
Impaired loans with reserves	56,449			73,438
Total Impaired Loans	61,548	[1]		82,017	[2]
Allowance for loan losses	9,879		10,401	11,912		11,226	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	6,496		5,231	5,231	5,622
Preferred Equity Interests
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	19,330			19,334
Impaired loans with reserves	4,060			4,180
Total Impaired Loans	23,390	[1]		23,514	[2]
Allowance for loan losses	$ 3,770		$ 3,830	$ 3,928		$ 242	$ 242	$ 242

[1] As of June 30, 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.