Delinquency Statistics of Commercial Real Estate Loan (Detail) - USD ($) $ in Thousands	Jun. 30, 2017		Dec. 31, 2016
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	$ 1,271,253		$ 1,294,874
Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	1,214,797		1,243,160
30 to 59 days
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total			50,335
90 days or more
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	56,456		1,379
Non-accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	122,054	[1]	121,038	[2]
Commercial Mortgage Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	1,158,504		1,162,233
Commercial Mortgage Loans | Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	1,107,427		1,111,898
Commercial Mortgage Loans | 30 to 59 days
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total			50,335
Commercial Mortgage Loans | 90 days or more
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	51,077
Commercial Mortgage Loans | Non-accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	104,723	[1]	113,509	[2]
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	72,420		89,811
Mezzanine Loans | Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	67,041		88,432
Mezzanine Loans | 90 days or more
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	5,379		1,379
Mezzanine Loans | Non-accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	13,681	[1]	1,379	[2]
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	40,329		42,830
Preferred Equity Interests | Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	40,329		42,830
Preferred Equity Interests | Non-accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	$ 3,650	[1]	$ 6,150	[2]

[1] Includes five loans that were current and one loan that is 60 to 89 days past due in accordance with their terms, but are on non-accrual due to uncertainty over whether we will fully collect principal and interest.

[2] Includes five loans that are current and one loan that is 30 to 59 days past due in accordance with their terms, but are on non-accrual due to uncertainty over whether we will fully collect principal and interest.