Investments in Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018		Dec. 31, 2017
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	$ 38,394
Commercial Portfolio Segment
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	1,049,058	[1]	$ 1,272,951	[2]
Commercial Portfolio Segment | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	101,760		106,136
Commercial Portfolio Segment | Commercial mortgage loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	992,356		1,194,016
Commercial Portfolio Segment | Commercial mortgage loans | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	50,990		55,353
Commercial Portfolio Segment | Commercial mortgage loans | Conduit Loans
Mortgage Loans On Real Estate [Line Items]
Deferred fees	6,485		8,497
Deferred costs	4,711		6,153
Commercial Portfolio Segment | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	23,422		45,652
Commercial Portfolio Segment | Mezzanine Loans | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	21,139		21,149
Commercial Portfolio Segment | Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	33,280		33,283
Commercial Portfolio Segment | Preferred Equity Interests | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	$ 29,631		$ 29,634

[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.