Investment in Loans by Credit Risk Category (Detail) - USD ($) $ in Thousands	Mar. 31, 2018		Dec. 31, 2017
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 90,260
Commercial Portfolio Segment
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,049,556	[1]	$ 1,273,507	[2]
Commercial Portfolio Segment | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	993,018		1,194,735
Commercial Portfolio Segment | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	23,257		45,488
Commercial Portfolio Segment | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	33,281		33,284
Commercial Portfolio Segment | Satisfactory
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	903,980		1,124,729
Commercial Portfolio Segment | Satisfactory | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	887,932		1,083,741
Commercial Portfolio Segment | Satisfactory | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	2,117		19,109
Commercial Portfolio Segment | Satisfactory | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	13,931		21,879
Commercial Portfolio Segment | Watchlist
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	145,576	[3]	148,778	[4]
Commercial Portfolio Segment | Watchlist | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	105,086	[3]	110,994	[4]
Commercial Portfolio Segment | Watchlist | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	21,140	[3]	26,379	[4]
Commercial Portfolio Segment | Watchlist | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 19,350	[3]	$ 11,405	[4]

[1] Includes $101,760 of cash flow loans, of which $50,990 are commercial mortgage loans, $21,139 are mezzanine loans and $29,631 are preferred equity interests. See Note 2: Summary of Significant Accounting Policies, (j) Revenue Recognition, for further discussion of our cash flow loans.

[2] Includes $106,136 of cash flow loans, of which $55,353 are commercial mortgage loans, $21,149 are mezzanine loans and $29,634 are preferred equity interests. See Note 2: Summary of Significant Accounting Policies, (k) Revenue Recognition, for further discussion of our cash flow loans.

[3] Includes $123,276 of loans that are considered to be impaired and $22,300 of loans that are not impaired.

[4] Includes $126,478 of loans that are considered to be impaired and $22,300 of loans that are not considered impaired.