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, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	107	110
Net book value	$ 12,214,565	$ 10,056,732
Total Loan Exposure	$ 12,792,919	$ 11,093,608
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	49	53
Net book value	$ 5,938,663	$ 5,773,972
Total Loan Exposure	$ 5,990,592	$ 5,807,170
Percentage of Portfolio	47.00%	53.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	14	15
Net book value	$ 2,211,798	$ 1,830,568
Total Loan Exposure	$ 2,292,082	$ 1,905,497
Percentage of Portfolio	18.00%	17.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	27	6
Net book value	$ 1,553,570	$ 487,473
Total Loan Exposure	$ 1,561,517	$ 940,980
Percentage of Portfolio	12.00%	8.00%
Spanish Assets [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,159,765
Total Loan Exposure	$ 1,168,400
Percentage of Portfolio	9.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	25
Net book value	$ 423,951	$ 1,220,423
Total Loan Exposure	$ 424,841	$ 1,228,959
Percentage of Portfolio	3.00%	11.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 124,777	$ 142,342
Total Loan Exposure	$ 216,634	$ 268,751
Percentage of Portfolio	2.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	9
Net book value	$ 802,041	$ 601,954
Total Loan Exposure	$ 1,138,853	$ 942,251
Percentage of Portfolio	9.00%	9.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	33	26
Net book value	$ 4,188,084	$ 2,857,948
Total Loan Exposure	$ 4,226,766	$ 2,871,219
Percentage of Portfolio	32.00%	26.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	18	17
Net book value	$ 2,121,488	$ 2,007,202
Total Loan Exposure	$ 2,133,466	$ 2,470,992
Percentage of Portfolio	17.00%	22.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	24	29
Net book value	$ 2,355,486	$ 2,672,069
Total Loan Exposure	$ 2,463,864	$ 2,816,276
Percentage of Portfolio	19.00%	24.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	6	9
Net book value	$ 827,600	$ 856,559
Total Loan Exposure	$ 833,003	$ 862,578
Percentage of Portfolio	7.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	11	10
Net book value	$ 505,127	$ 380,204
Total Loan Exposure	$ 508,564	$ 380,120
Percentage of Portfolio	4.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	2
Net book value	$ 128,109	$ 283,381
Total Loan Exposure	$ 128,607	$ 286,221
Percentage of Portfolio	1.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	93	93
Net book value	$ 10,125,894	$ 9,057,363
Total Loan Exposure	$ 10,294,270	$ 9,687,406
Percentage of Portfolio	80.00%	86.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,159,765
Total Loan Exposure	$ 1,168,400
Percentage of Portfolio	9.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	6
Net book value	$ 499,028	$ 440,317
Total Loan Exposure	$ 848,918	$ 794,789
Percentage of Portfolio	7.00%	7.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	7
Net book value	$ 331,975	$ 415,893
Total Loan Exposure	$ 329,004	$ 412,343
Percentage of Portfolio	3.00%	4.00%
Belgium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 71,926	$ 73,779
Total Loan Exposure	$ 72,441	$ 74,431
Percentage of Portfolio	1.00%	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 11,958	$ 12,237
Total Loan Exposure	$ 65,864	$ 67,399
Percentage of Portfolio		1.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	2
Net book value	$ 14,019	$ 57,143
Total Loan Exposure	$ 14,022	$ 57,240
Percentage of Portfolio		1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	14	17
Net book value	$ 2,088,671	$ 999,369
Total Loan Exposure	$ 2,498,649	$ 1,406,202
Percentage of Portfolio	20.00%	14.00%