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	Mar. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,943,383	$ 4,428,500
Senior Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	4,909,214	4,340,586
Percentage of Book Value	99.00%	98.00%
Subordinate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	34,169	87,914
Percentage of Book Value	1.00%	2.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	4,943,383	4,428,500
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	2,516,133	1,878,605
Percentage of Book Value	51.00%	42.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,269,408	1,267,486
Percentage of Book Value	26.00%	29.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	414,807	426,094
Percentage of Book Value	8.00%	10.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	252,065	315,686
Percentage of Book Value	5.00%	7.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	218,620	270,812
Percentage of Book Value	4.00%	6.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	272,350	269,817
Percentage of Book Value	6.00%	6.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,508,339	1,383,258
Percentage of Book Value	31.00%	31.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	756,791	628,275
Percentage of Book Value	15.00%	14.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	666,938	657,484
Percentage of Book Value	13.00%	15.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	416,234	138,796
Percentage of Book Value	8.00%	3.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	389,586	335,406
Percentage of Book Value	8.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	377,964	405,741
Percentage of Book Value	8.00%	9.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	4,115,852	3,548,960
Percentage of Book Value	83.00%	80.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	603,306	622,692
Percentage of Book Value	12.00%	14.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	117,355	137,024
Percentage of Book Value	2.00%	3.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	76,954	86,289
Percentage of Book Value	2.00%	2.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	29,916	33,535
Percentage of Book Value	1.00%	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 827,531	$ 879,540
Percentage of Book Value	17.00%	20.00%