Investment in Loans by Credit Risk Category (Detail) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 1,273,507
Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,194,735
Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	45,488
Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	33,284
Commercial Portfolio Segment
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,273,507	[1]	$ 1,294,874	[2]
Commercial Portfolio Segment | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,194,735		1,162,233	[3]
Commercial Portfolio Segment | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	45,488		89,811
Commercial Portfolio Segment | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	33,284		42,830
Commercial Portfolio Segment | Satisfactory
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,124,729		1,142,467
Commercial Portfolio Segment | Satisfactory | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,083,741		1,048,724
Commercial Portfolio Segment | Satisfactory | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	19,109		57,063
Commercial Portfolio Segment | Satisfactory | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	21,879		36,680
Commercial Portfolio Segment | Watchlist
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	148,778	[4]	152,407	[5]
Commercial Portfolio Segment | Watchlist | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	110,994	[4]	113,509	[5]
Commercial Portfolio Segment | Watchlist | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	26,379	[4]	32,748	[5]
Commercial Portfolio Segment | Watchlist | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 11,405	[4]	$ 6,150	[5]

[1] 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.

[2] Includes $155,750 of cash flow loans, of which $98,784 are commercial mortgage loans, $21,171 are mezzanine loans and $35,795 are preferred equity interests. See Note 2: Summary of Significant Accounting Policies, (k) Revenue Recognition, for further discussion of our cash flow loans.

[3] Commercial mortgage loans include two conduit loans with unpaid principal balances and carrying amounts totaling $20,181, a weighted-average coupon of 4.8%, and maturity dates ranging from June 2025 through December 2025. These commercial mortgage loans are accounted for as loans held for sale

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

[5] Includes $152,407 of loans that are considered to be impaired.