Loans Held for Investment (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019		Dec. 31, 2018
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	$ 6,967
Commercial Portfolio Segment
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	438,641	[1]	$ 503,071	[2]
Commercial Portfolio Segment | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	53,128		54,621
Commercial Portfolio Segment | Commercial mortgage loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	388,849		453,217
Commercial Portfolio Segment | Commercial mortgage loans | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	7,148		8,579
Commercial Portfolio Segment | Commercial mortgage loans | Conduit Loans
Mortgage Loans On Real Estate [Line Items]
Deferred fees	1,703		2,558
Deferred costs	1,306		1,884
Commercial Portfolio Segment | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	21,278		21,278
Commercial Portfolio Segment | Mezzanine Loans | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	21,114		21,114
Commercial Portfolio Segment | Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	28,514		28,576
Commercial Portfolio Segment | Preferred Equity Interests | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount	$ 24,866		$ 24,928

[1] Includes $53,128 of cash flow loans, of which $7,148 are commercial mortgage loans, $21,114 are mezzanine loans and $24,866 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 $54,621 of cash flow loans, of which $8,579 are commercial mortgage loans, $21,114 are mezzanine loans and $24,928 are preferred equity interests. See Note 2: Summary of Significant Accounting Policies, (j) Revenue Recognition, for further discussion of our cash flow loans.