Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	102	107
Net book value	$ 9,587,457	$ 8,692,978
Total Loan Exposure	$ 10,613,660	$ 9,756,734
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	55	55
Net book value	$ 5,622,781	$ 4,800,609
Total Loan Exposure	$ 5,652,170	$ 4,889,456
Percentage of Portfolio	54.00%	50.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	14	18
Net book value	$ 2,068,196	$ 1,889,732
Total Loan Exposure	$ 2,141,347	$ 1,957,334
Percentage of Portfolio	20.00%	20.00%
Manufactured Housing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	9
Net book value	$ 233,201	$ 296,290
Total Loan Exposure	$ 232,971	$ 296,252
Percentage of Portfolio	2.00%	3.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	9
Net book value	$ 532,397	$ 769,813
Total Loan Exposure	$ 965,804	$ 1,173,592
Percentage of Portfolio	9.00%	12.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	10	8
Net book value	$ 639,622	$ 521,097
Total Loan Exposure	$ 644,211	$ 523,529
Percentage of Portfolio	6.00%	5.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	2
Net book value	$ 55,916	$ 66,070
Total Loan Exposure	$ 215,113	$ 258,360
Percentage of Portfolio	2.00%	3.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	6
Net book value	$ 435,344	$ 349,367
Total Loan Exposure	$ 762,044	$ 658,211
Percentage of Portfolio	7.00%	7.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	25	26
Net book value	$ 2,550,440	$ 2,548,257
Total Loan Exposure	$ 2,566,702	$ 2,562,149
Percentage of Portfolio	24.00%	26.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	21	21
Net book value	$ 2,025,144	$ 1,492,530
Total Loan Exposure	$ 2,467,193	$ 1,899,748
Percentage of Portfolio	23.00%	19.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	22	22
Net book value	$ 2,044,017	$ 1,628,811
Total Loan Exposure	$ 2,212,783	$ 1,828,667
Percentage of Portfolio	21.00%	19.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	7
Net book value	$ 877,698	$ 695,713
Total Loan Exposure	$ 881,658	$ 698,093
Percentage of Portfolio	8.00%	7.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	8
Net book value	$ 272,302	$ 380,639
Total Loan Exposure	$ 270,830	$ 379,766
Percentage of Portfolio	3.00%	4.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	3
Net book value	$ 236,134	$ 227,747
Total Loan Exposure	$ 238,522	$ 293,564
Percentage of Portfolio	2.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	83	87
Net book value	$ 8,005,735	$ 6,973,697
Total Loan Exposure	$ 8,637,688	$ 7,661,987
Percentage of Portfolio	81.00%	78.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	9
Net book value	$ 1,034,661	$ 977,136
Total Loan Exposure	$ 1,379,591	$ 1,305,816
Percentage of Portfolio	13.00%	13.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	8
Net book value	$ 480,906	$ 487,835
Total Loan Exposure	$ 476,843	$ 483,923
Percentage of Portfolio	4.00%	5.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 11,764	$ 204,241
Total Loan Exposure	$ 64,940	$ 254,644
Percentage of Portfolio	1.00%	3.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 54,391	$ 50,069
Total Loan Exposure	$ 54,598	$ 50,364
Percentage of Portfolio	1.00%	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	19	20
Net book value	$ 1,581,722	$ 1,719,281
Total Loan Exposure	$ 1,975,972	$ 2,094,747
Percentage of Portfolio	19.00%	22.00%