Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	111	107
Net book value	$ 9,637,152	$ 8,692,978
Total Loan Exposure	$ 10,668,677	$ 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	$ 5,781,675	$ 4,800,609
Total Loan Exposure	$ 5,814,214	$ 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	$ 1,713,162	$ 1,889,732
Total Loan Exposure	$ 1,784,893	$ 1,957,334
Percentage of Portfolio	17.00%	20.00%
Manufactured Housing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	9
Net book value	$ 232,148	$ 296,290
Total Loan Exposure	$ 231,856	$ 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	$ 539,752	$ 769,813
Total Loan Exposure	$ 982,270	$ 1,173,592
Percentage of Portfolio	9.00%	12.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	8
Net book value	$ 762,969	$ 521,097
Total Loan Exposure	$ 767,875	$ 523,529
Percentage of Portfolio	7.00%	5.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 129,421	$ 66,070
Total Loan Exposure	$ 273,112	$ 258,360
Percentage of Portfolio	3.00%	3.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	6
Net book value	$ 478,025	$ 349,367
Total Loan Exposure	$ 814,457	$ 658,211
Percentage of Portfolio	8.00%	7.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	26	26
Net book value	$ 2,680,546	$ 2,548,257
Total Loan Exposure	$ 2,694,018	$ 2,562,149
Percentage of Portfolio	25.00%	26.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	21	21
Net book value	$ 1,964,534	$ 1,492,530
Total Loan Exposure	$ 2,414,994	$ 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	28	22
Net book value	$ 2,470,097	$ 1,628,811
Total Loan Exposure	$ 2,629,456	$ 1,828,667
Percentage of Portfolio	24.00%	19.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	7
Net book value	$ 890,546	$ 695,713
Total Loan Exposure	$ 894,564	$ 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	8	8
Net book value	$ 291,792	$ 380,639
Total Loan Exposure	$ 290,393	$ 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	$ 249,118	$ 227,747
Total Loan Exposure	$ 251,422	$ 293,564
Percentage of Portfolio	2.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	93	87
Net book value	$ 8,546,633	$ 6,973,697
Total Loan Exposure	$ 9,174,847	$ 7,661,987
Percentage of Portfolio	85.00%	78.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	9
Net book value	$ 486,794	$ 977,136
Total Loan Exposure	$ 838,763	$ 1,305,816
Percentage of Portfolio	8.00%	13.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	8
Net book value	$ 462,832	$ 487,835
Total Loan Exposure	$ 458,619	$ 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	$ 12,114	$ 204,241
Total Loan Exposure	$ 66,810	$ 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	$ 56,235	$ 50,069
Total Loan Exposure	$ 56,391	$ 50,364
Percentage of Portfolio	1.00%	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	18	20
Net book value	$ 1,090,519	$ 1,719,281
Total Loan Exposure	$ 1,493,830	$ 2,094,747
Percentage of Portfolio	15.00%	22.00%
Belgium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 72,544
Total Loan Exposure	$ 73,247
Percentage of Portfolio	1.00%