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	Dec. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,428,500	$ 2,047,223
Senior Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	4,340,586	1,800,329
Percentage of Book Value	98.00%	88.00%
Subordinate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	87,914	246,894
Percentage of Book Value	2.00%	12.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	4,428,500	2,047,223
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,878,605	864,666
Percentage of Book Value	42.00%	42.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,267,486	390,492
Percentage of Book Value	29.00%	19.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	426,094	341,819
Percentage of Book Value	10.00%	17.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	315,686	275,645
Percentage of Book Value	7.00%	13.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	270,812	43,115
Percentage of Book Value	6.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	269,817	131,486
Percentage of Book Value	6.00%	7.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	1,383,258	828,571
Percentage of Book Value	31.00%	40.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	657,484	243,798
Percentage of Book Value	15.00%	12.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	628,275	469,262
Percentage of Book Value	14.00%	23.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	405,741	216,429
Percentage of Book Value	9.00%	11.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	335,406	85,708
Percentage of Book Value	8.00%	4.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	138,796	166,207
Percentage of Book Value	3.00%	8.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	3,548,960	2,009,975
Percentage of Book Value	80.00%	98.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	622,692	37,248
Percentage of Book Value	14.00%	2.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	137,024
Percentage of Book Value	3.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	86,289
Percentage of Book Value	2.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	33,535
Percentage of Book Value	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 879,540	$ 37,248
Percentage of Book Value	20.00%	2.00%