Loans Receivable - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	116	125
Net book value	$ 9,090,934	$ 9,077,007
Total Loan Exposure	$ 10,185,476	$ 10,148,126
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	54	55
Net book value	$ 4,110,928	$ 4,039,521
Total Loan Exposure	$ 4,176,738	$ 4,085,007
Percentage of Portfolio	41.00%	41.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	18	20
Net book value	$ 1,863,974	$ 1,903,544
Total Loan Exposure	$ 1,937,744	$ 1,986,113
Percentage of Portfolio	19.00%	20.00%
Manufactured Housing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	15	18
Net book value	$ 1,259,884	$ 1,361,572
Total Loan Exposure	$ 1,257,691	$ 1,359,132
Percentage of Portfolio	12.00%	13.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	9
Net book value	$ 818,471	$ 684,944
Total Loan Exposure	$ 1,195,197	$ 1,031,405
Percentage of Portfolio	12.00%	10.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	10	11
Net book value	$ 622,038	$ 580,112
Total Loan Exposure	$ 623,892	$ 582,545
Percentage of Portfolio	6.00%	6.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	3
Net book value	$ 101,238	$ 127,434
Total Loan Exposure	$ 344,337	$ 353,144
Percentage of Portfolio	3.00%	3.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	9
Net book value	$ 314,401	$ 379,880
Total Loan Exposure	$ 649,877	$ 750,780
Percentage of Portfolio	7.00%	7.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	23	25
Net book value	$ 2,158,020	$ 2,260,392
Total Loan Exposure	$ 2,168,623	$ 2,272,163
Percentage of Portfolio	21.00%	22.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	25	27
Net book value	$ 1,716,458	$ 1,836,766
Total Loan Exposure	$ 2,093,981	$ 2,185,609
Percentage of Portfolio	20.00%	21.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	20	22
Net book value	$ 1,311,546	$ 1,125,238
Total Loan Exposure	$ 1,560,118	$ 1,356,301
Percentage of Portfolio	15.00%	13.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	5
Net book value	$ 921,518	$ 616,964
Total Loan Exposure	$ 924,728	$ 617,774
Percentage of Portfolio	9.00%	6.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	12	15
Net book value	$ 877,497	$ 1,035,839
Total Loan Exposure	$ 875,685	$ 1,034,732
Percentage of Portfolio	9.00%	10.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	6	5
Net book value	$ 261,194	$ 390,307
Total Loan Exposure	$ 305,556	$ 415,207
Percentage of Portfolio	3.00%	4.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	93	99
Net book value	$ 7,246,233	$ 7,265,506
Total Loan Exposure	$ 7,928,691	$ 7,881,786
Percentage of Portfolio	77.00%	76.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	10	10
Net book value	$ 981,209	$ 888,998
Total Loan Exposure	$ 1,340,300	$ 1,283,644
Percentage of Portfolio	13.00%	13.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	11
Net book value	$ 518,592	$ 561,023
Total Loan Exposure	$ 515,379	$ 558,724
Percentage of Portfolio	5.00%	6.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	2
Net book value	$ 218,458	$ 235,294
Total Loan Exposure	$ 273,503	$ 296,424
Percentage of Portfolio	3.00%	3.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 67,930	$ 66,661
Total Loan Exposure	$ 68,600	$ 67,416
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	$ 58,512	$ 59,525
Total Loan Exposure	$ 59,003	$ 60,132
Percentage of Portfolio	1.00%	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	23	26
Net book value	$ 1,844,701	$ 1,811,501
Total Loan Exposure	$ 2,256,785	$ 2,266,340
Percentage of Portfolio	23.00%	24.00%