Loans Considered to be Impaired (Detail) - USD ($) $ in Thousands	Sep. 30, 2018		Jun. 30, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	$ 35,032			$ 34,946
Impaired loans with reserves	119,549			91,532
Total Impaired Loans	154,581	[1]		126,478	[2]
Allowance for loan losses	23,617		$ 16,699	14,883		$ 25,140	$ 23,514	$ 12,354
Commercial Mortgage Loans
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	11,164			14,321
Impaired loans with reserves	102,946			74,372
Total Impaired Loans	114,110	[1]		88,693	[2]
Allowance for loan losses	16,161		11,226	9,019		16,090	12,722	10,640
Mezzanine Loans
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	12,869			12,869
Impaired loans with reserves	8,253			13,510
Total Impaired Loans	21,122	[1]		26,379	[2]
Allowance for loan losses	6,489		5,231	5,622		7,350	9,092
Preferred Equity Interests
Financing Receivable Impaired [Line Items]
Impaired loans expecting full recovery	10,999			7,756
Impaired loans with reserves	8,350			3,650
Total Impaired Loans	19,349	[1]		11,406	[2]
Allowance for loan losses	$ 967		$ 242	$ 242		$ 1,700	$ 1,700	$ 1,714

[1] As of September 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.