Investment in Loans by Credit Risk Category (Detail) - USD ($) $ in Thousands	Sep. 30, 2017		Dec. 31, 2016
Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 18,500
Commercial Portfolio Segment
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,263,928	[1]	$ 1,294,874	[2]
Commercial Portfolio Segment | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,158,089	[3]	1,162,233	[4]
Commercial Portfolio Segment | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	65,512		89,811
Commercial Portfolio Segment | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	40,327		42,830
Commercial Portfolio Segment | Satisfactory
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,082,820		1,142,467
Commercial Portfolio Segment | Satisfactory | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	1,030,065		1,048,724
Commercial Portfolio Segment | Satisfactory | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	19,133		57,063
Commercial Portfolio Segment | Satisfactory | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	33,622		36,680
Commercial Portfolio Segment | Watchlist
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	181,108	[5]	152,407	[6]
Commercial Portfolio Segment | Watchlist | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	128,024	[5]	113,509	[6]
Commercial Portfolio Segment | Watchlist | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	46,379	[5]	32,748	[6]
Commercial Portfolio Segment | Watchlist | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 6,705	[5]	$ 6,150	[6]

[1] Includes $139,007 of cash flow loans, of which $82,055 are commercial mortgage loans, $21,159 are mezzanine loans and $35,793 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 $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, (j) Revenue Recognition, for further discussion of our cash flow loans.

[3] Commercial mortgage loans include one conduit loan with an unpaid principal balance and carrying amount of $16,725, a weighted-average coupon of 4.8%, and a maturity date of December 2025. This commercial mortgage loan is accounted for as a loan held for sale.

[4] 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.

[5] Includes $158,808 of loans that are considered to be impaired and $22,300 of loans that are not considered to be impaired.

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