Loans Receivable, Net - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	106	110
Net book value	$ 11,081,716	$ 10,056,732
Total Loan Exposure	$ 12,136,883	$ 11,093,608
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	52	53
Net book value	$ 5,725,784	$ 5,773,972
Total Loan Exposure	$ 5,756,444	$ 5,807,170
Percentage of Portfolio	48.00%	53.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	14	15
Net book value	$ 1,569,512	$ 1,830,568
Total Loan Exposure	$ 1,646,238	$ 1,905,497
Percentage of Portfolio	14.00%	17.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	25	6
Net book value	$ 1,481,524	$ 487,473
Total Loan Exposure	$ 1,489,938	$ 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	$ 1,220,214
Total Loan Exposure	$ 1,230,000
Percentage of Portfolio	10.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	25
Net book value	$ 446,482	$ 1,220,423
Total Loan Exposure	$ 908,160	$ 1,228,959
Percentage of Portfolio	7.00%	11.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 149,137	$ 142,342
Total Loan Exposure	$ 262,175	$ 268,751
Percentage of Portfolio	2.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	9
Net book value	$ 489,063	$ 601,954
Total Loan Exposure	$ 843,928	$ 942,251
Percentage of Portfolio	7.00%	9.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	27	26
Net book value	$ 3,179,789	$ 2,857,948
Total Loan Exposure	$ 3,191,691	$ 2,871,219
Percentage of Portfolio	26.00%	26.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	17
Net book value	$ 2,048,670	$ 2,007,202
Total Loan Exposure	$ 2,520,778	$ 2,470,992
Percentage of Portfolio	21.00%	22.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	27	29
Net book value	$ 2,322,490	$ 2,672,069
Total Loan Exposure	$ 2,451,827	$ 2,816,276
Percentage of Portfolio	20.00%	24.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	9
Net book value	$ 753,689	$ 856,559
Total Loan Exposure	$ 759,148	$ 862,578
Percentage of Portfolio	6.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	10
Net book value	$ 309,161	$ 380,204
Total Loan Exposure	$ 310,928	$ 380,120
Percentage of Portfolio	3.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	2
Net book value	$ 264,167	$ 283,381
Total Loan Exposure	$ 266,724	$ 286,221
Percentage of Portfolio	2.00%	3.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	89	93
Net book value	$ 8,877,966	$ 9,057,363
Total Loan Exposure	$ 9,501,096	$ 9,687,406
Percentage of Portfolio	78.00%	86.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 1,220,214
Total Loan Exposure	$ 1,230,000
Percentage of Portfolio	10.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	6	6
Net book value	$ 458,081	$ 440,317
Total Loan Exposure	$ 825,497	$ 794,789
Percentage of Portfolio	7.00%	7.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	6	7
Net book value	$ 389,147	$ 415,893
Total Loan Exposure	$ 386,131	$ 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	$ 75,655	$ 73,779
Total Loan Exposure	$ 76,260	$ 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	$ 12,671	$ 12,237
Total Loan Exposure	$ 69,891	$ 67,399
Percentage of Portfolio	1.00%	1.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 47,982	$ 57,143
Total Loan Exposure	$ 48,008	$ 57,240
Percentage of Portfolio		1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	17
Net book value	$ 2,203,750	$ 999,369
Total Loan Exposure	$ 2,635,787	$ 1,406,202
Percentage of Portfolio	22.00%	14.00%