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	Jun. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 10,131,323	$ 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	$ 9,977,017	$ 4,340,586
Percentage of Book Value	98.00%	98.00%
Subordinate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 154,306	$ 87,914
Percentage of Book Value	2.00%	2.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 10,131,323	$ 4,428,500
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,088,256	$ 1,878,605
Percentage of Book Value	40.00%	42.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 2,231,682	$ 1,267,486
Percentage of Book Value	22.00%	29.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 472,867	$ 426,094
Percentage of Book Value	5.00%	10.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 352,152	$ 315,686
Percentage of Book Value	3.00%	7.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 773,034	$ 270,812
Percentage of Book Value	8.00%	6.00%
Manufactured Housing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,415,800
Percentage of Book Value	14.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 797,532	$ 269,817
Percentage of Book Value	8.00%	6.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 2,275,132	$ 1,383,258
Percentage of Book Value	22.00%	31.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,293,416	$ 628,275
Percentage of Book Value	13.00%	14.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,952,500	$ 657,484
Percentage of Book Value	19.00%	15.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 398,951	$ 138,796
Percentage of Book Value	4.00%	3.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 607,835	$ 335,406
Percentage of Book Value	6.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,266,544	$ 405,741
Percentage of Book Value	13.00%	9.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 7,794,378	$ 3,548,960
Percentage of Book Value	77.00%	80.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,159,450	$ 622,692
Percentage of Book Value	11.00%	14.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 806,010	$ 137,024
Percentage of Book Value	8.00%	3.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 79,875	$ 86,289
Percentage of Book Value	1.00%	2.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 61,045	$ 33,535
Percentage of Book Value	1.00%	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 230,565
Percentage of Book Value	2.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 2,336,945	$ 879,540
Percentage of Book Value	23.00%	20.00%