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	Sep. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,377,591	$ 4,428,500
Percentage of Book Value	100.00%	100.00%
Senior Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,118,863	$ 4,340,586
Percentage of Book Value	97.00%	98.00%
Subordinate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 258,728	$ 87,914
Percentage of Book Value	3.00%	2.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,377,591	$ 4,428,500
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 3,894,843	$ 1,878,605
Percentage of Book Value	41.00%	42.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,943,233	$ 1,267,486
Percentage of Book Value	21.00%	29.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 581,689	$ 426,094
Percentage of Book Value	6.00%	10.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 163,323	$ 315,686
Percentage of Book Value	2.00%	7.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 762,293	$ 270,812
Percentage of Book Value	8.00%	6.00%
Manufactured Housing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,407,890
Percentage of Book Value	15.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 624,320	$ 269,817
Percentage of Book Value	7.00%	6.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 2,220,088	$ 1,383,258
Percentage of Book Value	23.00%	31.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,920,611	$ 657,484
Percentage of Book Value	20.00%	15.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,211,043	$ 405,741
Percentage of Book Value	13.00%	9.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,105,932	$ 628,275
Percentage of Book Value	12.00%	14.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 615,121	$ 335,406
Percentage of Book Value	7.00%	8.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 384,855	$ 138,796
Percentage of Book Value	4.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 7,457,650	$ 3,548,960
Percentage of Book Value	79.00%	80.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 970,099	$ 622,692
Percentage of Book Value	10.00%	14.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 577,181	$ 137,024
Percentage of Book Value	6.00%	3.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 68,398	$ 86,289
Percentage of Book Value	1.00%	2.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 61,181	$ 33,535
Percentage of Book Value	1.00%	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 243,082
Percentage of Book Value	3.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,919,941	$ 879,540
Percentage of Book Value	21.00%	20.00%