Loans Receivable - Types of Loans in Loan Portfolio, as Well as Property Type and Geographic Distribution of Properties Securing these Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,362,891	$ 141,500
Senior Mortgages Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,285,891	62,500
Percentage of Book Value	94.00%	44.00%
Subordinated Interests in Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	77,000	79,000
Percentage of Book Value	6.00%	56.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,362,891	141,500
Loans Receivable [Member] | Asset [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,362,891	141,500
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	614,025	111,500
Percentage of Book Value	45.00%	79.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	383,601
Percentage of Book Value	28.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	257,942	30,000
Percentage of Book Value	19.00%	21.00%
Land [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	64,271
Percentage of Book Value	5.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	43,052
Percentage of Book Value	3.00%
West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	402,121	92,500
Percentage of Book Value	30.00%	65.00%
Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	523,662	27,000
Percentage of Book Value	38.00%	19.00%
Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	219,093	12,404
Percentage of Book Value	16.00%	9.00%
Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	85,608
Percentage of Book Value	6.00%
Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 132,407	$ 9,596
Percentage of Book Value	10.00%	7.00%