Loans Held for Investment by Credit Risk Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2018		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	$ 13,000		$ 90,260
Commercial Portfolio Segment
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	733,065	[1]		$ 1,273,507	[2]
Commercial Portfolio Segment | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	676,654			1,194,735
Commercial Portfolio Segment | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	23,131			45,488
Commercial Portfolio Segment | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	33,280			33,284
Commercial Portfolio Segment | Satisfactory
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	582,158			1,124,729
Commercial Portfolio Segment | Satisfactory | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	566,228			1,083,741
Commercial Portfolio Segment | Satisfactory | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	2,000			19,109
Commercial Portfolio Segment | Satisfactory | Preferred Equity Interests
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	13,930			21,879
Commercial Portfolio Segment | Watchlist
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	150,907	[3]		148,778	[4]
Commercial Portfolio Segment | Watchlist | Commercial Mortgage Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	110,426	[3]		110,994	[4]
Commercial Portfolio Segment | Watchlist | Mezzanine Loans
Financing Receivable Recorded Investment [Line Items]
Unpaid Principal Balance	21,131	[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 $96,104 of cash flow loans, of which $45,343 are commercial mortgage loans, $21,130 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, (j) Revenue Recognition, for further discussion of our cash flow loans.

[3] Includes $128,467 of loans that are considered to be impaired and $22,440 of loans that are not considered to be impaired.

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