Loans Considered to be Impaired (Detail) - USD ($) $ in Thousands	Jun. 30, 2018		Mar. 31, 2018	Dec. 31, 2017		Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	$ 42,678			$ 34,946
Impaired loans with reserves	85,789			91,532
Total Impaired Loans	128,467	[1]		126,478	[2]
Allowance for loan losses	16,699		$ 13,732	14,883		$ 23,514	$ 13,531	$ 12,354
Commercial Mortgage Loans
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	14,108			14,321
Impaired loans with reserves	73,878			74,372
Total Impaired Loans	87,986	[1]		88,693	[2]
Allowance for loan losses	11,226		8,259	9,019		12,722	10,979	10,640
Mezzanine Loans
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	12,870			12,869
Impaired loans with reserves	8,261			13,510
Total Impaired Loans	21,131	[1]		26,379	[2]
Allowance for loan losses	5,231		5,231	5,622		9,092	852
Preferred Equity Interests
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	15,700			7,756
Impaired loans with reserves	3,650			3,650
Total Impaired Loans	19,350	[1]		11,406	[2]
Allowance for loan losses	$ 242		$ 242	$ 242		$ 1,700	$ 1,700	$ 1,714

[1] As of June 30, 2018, there was no unpaid principal relating to previously identified TDRs that are on accrual status.

[2] As of December 31, 2017, there was no unpaid principal relating to previously identified TDRs that are on accrual status.