Loans Receivable - Property Type and Geographic Distribution of Properties Securing Loans in Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	106	125
Net book value	$ 8,347,712	$ 9,077,007
Total Loan Exposure	$ 9,437,543	$ 10,148,126
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	$ 4,540,394	$ 4,039,521
Total Loan Exposure	$ 4,610,800	$ 4,085,007
Percentage of Portfolio	50.00%	41.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	17	20
Net book value	$ 1,761,751	$ 1,903,544
Total Loan Exposure	$ 1,832,501	$ 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	9	18
Net book value	$ 297,726	$ 1,361,572
Total Loan Exposure	$ 297,847	$ 1,359,132
Percentage of Portfolio	3.00%	13.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	10	9
Net book value	$ 823,184	$ 684,944
Total Loan Exposure	$ 1,215,822	$ 1,031,405
Percentage of Portfolio	13.00%	10.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	11
Net book value	$ 586,910	$ 580,112
Total Loan Exposure	$ 588,809	$ 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	$ 85,436	$ 127,434
Total Loan Exposure	$ 315,816	$ 353,144
Percentage of Portfolio	3.00%	3.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	9
Net book value	$ 252,311	$ 379,880
Total Loan Exposure	$ 575,948	$ 750,780
Percentage of Portfolio	6.00%	7.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	25	25
Net book value	$ 2,449,223	$ 2,260,392
Total Loan Exposure	$ 2,461,548	$ 2,272,163
Percentage of Portfolio	25.00%	22.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	20	27
Net book value	$ 1,343,942	$ 1,836,766
Total Loan Exposure	$ 1,739,465	$ 2,185,609
Percentage of Portfolio	18.00%	21.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	21	22
Net book value	$ 1,496,295	$ 1,125,238
Total Loan Exposure	$ 1,733,491	$ 1,356,301
Percentage of Portfolio	18.00%	13.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	6	5
Net book value	$ 618,939	$ 616,964
Total Loan Exposure	$ 621,012	$ 617,774
Percentage of Portfolio	7.00%	6.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	15
Net book value	$ 444,381	$ 1,035,839
Total Loan Exposure	$ 443,653	$ 1,034,732
Percentage of Portfolio	5.00%	10.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	5
Net book value	$ 245,934	$ 390,307
Total Loan Exposure	$ 293,039	$ 415,207
Percentage of Portfolio	3.00%	4.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	85	99
Net book value	$ 6,598,714	$ 7,265,506
Total Loan Exposure	$ 7,292,208	$ 7,881,786
Percentage of Portfolio	76.00%	76.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	10
Net book value	$ 930,677	$ 888,998
Total Loan Exposure	$ 1,276,234	$ 1,283,644
Percentage of Portfolio	14.00%	13.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	11
Net book value	$ 477,466	$ 561,023
Total Loan Exposure	$ 474,182	$ 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	$ 218,855	$ 235,294
Total Loan Exposure	$ 271,922	$ 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	$ 68,572	$ 66,661
Total Loan Exposure	$ 69,198	$ 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	$ 53,428	$ 59,525
Total Loan Exposure	$ 53,799	$ 60,132
Percentage of Portfolio	1.00%	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	21	26
Net book value	$ 1,748,998	$ 1,811,501
Total Loan Exposure	$ 2,145,335	$ 2,266,340
Percentage of Portfolio	24.00%	24.00%