Investments in Loans (Parenthetical) (Detail) $ in Thousands		9 Months Ended		12 Months Ended
		Sep. 30, 2017 USD ($) Loan		Dec. 31, 2016 USD ($) Loan
Commercial mortgage loans
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Unpaid Principal Balance		$ 18,500
Commercial Portfolio Segment
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		124	[1]	134	[2]
Unpaid Principal Balance		$ 1,263,928	[1]	$ 1,294,874	[2]
Weighted-Average Coupon	[3]	5.90%	[1]	6.10%	[2]
Carrying Amount		$ 1,263,198	[1]	$ 1,294,066	[2]
Commercial Portfolio Segment | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount		$ 139,007		$ 155,750
Commercial Portfolio Segment | Commercial mortgage loans
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		91	[4]	94	[5]
Unpaid Principal Balance		$ 1,158,089	[4]	$ 1,162,233	[5]
Weighted-Average Coupon	[3]	5.50%	[4]	5.80%	[5]
Carrying Amount		$ 1,157,315	[4]	$ 1,161,381	[5]
Commercial Portfolio Segment | Commercial mortgage loans | Conduit Loans
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		1		2
Unpaid Principal Balance		$ 16,725		$ 20,181
Weighted-Average Coupon		4.80%		4.80%
Deferred fees		$ 8,482		$ 5,978
Deferred costs		5,630		4,551
Commercial Portfolio Segment | Commercial mortgage loans | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount		$ 82,055		$ 98,784
Commercial Portfolio Segment | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		16		23
Unpaid Principal Balance		$ 65,512		$ 89,811
Weighted-Average Coupon	[3]	10.10%		9.90%
Carrying Amount		$ 65,557		$ 89,856
Commercial Portfolio Segment | Mezzanine Loans | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount		$ 21,159		$ 21,171
Commercial Portfolio Segment | Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Number of Loans | Loan		17		17
Unpaid Principal Balance		$ 40,327		$ 42,830
Weighted-Average Coupon	[3]	8.70%		8.20%
Carrying Amount		$ 40,326		$ 42,829
Commercial Portfolio Segment | Preferred Equity Interests | Cash Flow Loans
Mortgage Loans On Real Estate [Line Items]
Carrying Amount		$ 35,793		$ 35,795

[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] Weighted-average coupon is calculated on the unpaid principal balance, which does not necessarily correspond to the carrying amount.

[4] 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.

[5] 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.