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	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,077,007	$ 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	$ 8,847,044	$ 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	$ 229,963	$ 87,914
Percentage of Book Value	3.00%	2.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,077,007	$ 4,428,500
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,039,521	$ 1,878,605
Percentage of Book Value	45.00%	42.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,903,544	$ 1,267,486
Percentage of Book Value	21.00%	29.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 580,112	$ 426,094
Percentage of Book Value	6.00%	10.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 127,434	$ 315,686
Percentage of Book Value	1.00%	7.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 684,944	$ 270,812
Percentage of Book Value	8.00%	6.00%
Manufactured Housing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,361,572
Percentage of Book Value	15.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 379,880	$ 269,817
Percentage of Book Value	4.00%	6.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 2,260,392	$ 1,383,258
Percentage of Book Value	25.00%	31.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,836,766	$ 657,484
Percentage of Book Value	20.00%	15.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,125,238	$ 628,275
Percentage of Book Value	12.00%	14.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,035,839	$ 405,741
Percentage of Book Value	11.00%	9.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 616,964	$ 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	$ 390,307	$ 138,796
Percentage of Book Value	4.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 7,265,506	$ 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	$ 888,998	$ 622,692
Percentage of Book Value	10.00%	14.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 561,023	$ 137,024
Percentage of Book Value	6.00%	3.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 66,661	$ 86,289
Percentage of Book Value	1.00%	2.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 59,525	$ 33,535
Percentage of Book Value	1.00%	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 235,294
Percentage of Book Value	3.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,811,501	$ 879,540
Percentage of Book Value	21.00%	20.00%