Delinquency Statistics of Commercial Real Estate Loan (Detail) - USD ($) $ in Thousands	Mar. 31, 2018		Dec. 31, 2017
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	$ 1,049,556		$ 1,273,507
Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	993,018		1,194,735
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	23,257		45,488
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	33,281		33,284
Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	974,303		1,223,043
Current | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	917,765		1,149,501
Current | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	23,257		40,258
Current | Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	33,281		33,284
30 to 59 days
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	30,146
30 to 59 days | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	30,146
90 days or more
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	45,107		50,464
90 days or more | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	45,107		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	87,456	[1]	98,603	[2]
Non-accrual | Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	75,536	[1]	81,443	[2]
Non-accrual | Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	8,270	[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 four loans that are current and two loans that are 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 one loan that is 90 days or more past due in accordance with their terms.