Delinquency Statistics of Commercial Real Estate Loan (Detail) - USD ($) $ in Thousands		Mar. 31, 2016		Dec. 31, 2015
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		$ 1,614,712		$ 1,627,121
Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		1,535,604		1,624,710
30 to 59 days
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		76,945
90 days or more
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		1,163		1,163
In foreclosure or bankruptcy proceedings
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	[1]	1,000		1,248
Non-Accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		35,689	[2]	35,937	[3]
Commercial mortgages
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		1,421,060		1,427,328
Commercial mortgages | Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		1,346,260		1,427,328
Commercial mortgages | 30 to 59 days
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		74,800
Commercial mortgages | Non-Accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		15,645	[2]	15,645	[3]
Mezzanine Loans
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		169,415		169,556
Mezzanine Loans | Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		165,107		167,145
Mezzanine Loans | 30 to 59 days
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		2,145
Mezzanine Loans | 90 days or more
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		1,163		1,163
Mezzanine Loans | In foreclosure or bankruptcy proceedings
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total	[1]	1,000		1,248
Mezzanine Loans | Non-Accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		12,098	[2]	12,346	[3]
Preferred Equity Interests
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		24,237		30,237
Preferred Equity Interests | Current
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		24,237		30,237
Preferred Equity Interests | Non-Accrual
Mortgage Loans On Real Estate [Line Items]
Delinquency statistics, Total		$ 7,946	[2]	$ 7,946	[3]

[1] These loans were on non-accrual due to uncertainty over full collection of principal and interest.

[2] Includes four loans that were current and two loans that were 30 days past due in accordance with their terms, however are on non-accrual due to uncertainty over full collection of principal and interest.

[3] Includes six loans that were current in accordance with their terms, however are on non-accrual due to uncertainty over full collection of principal and interest.