Delinquency Statistics of Commercial Real Estate Loan Held for Investment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018		Dec. 31, 2017
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	$ 733,065		$ 1,273,507
Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	676,654		1,194,735
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	23,131		45,488
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	33,280		33,284
Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	672,587		1,223,043
Current | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	616,176		1,149,501
Current | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	23,131		40,258
Current | Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	33,280		33,284
30 to 59 days
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	20,572
30 to 59 days | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	20,572
90 days or more
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	39,906		50,464
90 days or more | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	39,906		45,234
90 days or more | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total			5,230
Non-accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	92,647	[1]	98,603	[2]
Non-accrual | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	80,736	[1]	81,443	[2]
Non-accrual | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	8,261	[1]	13,510	[2]
Non-accrual | Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	$ 3,650	[1]	$ 3,650	[2]

[1] Includes six loans that are current and one loan that is 30 to 59 days or more past due, but are on non-accrual due to uncertainty over whether we will fully collect principal and interest. Also includes one loan that is 90 days or more past due in accordance with its terms

[2] Includes five loans that were current in accordance with their terms, but are on non-accrual due to uncertainty over whether we will fully collect principal and interest, and two loans that are 90 days or more past due in accordance with their terms.