Loans Receivable - Types of Loans in Loan Portfolio, as well as Property Type and Geographic Distribution of Properties Securing these Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,313,763	$ 9,077,007
Percentage of Book Value	100.00%	100.00%
Senior Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,076,146	$ 8,847,044
Percentage of Book Value	97.00%	97.00%
Subordinate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 237,617	$ 229,963
Percentage of Book Value	3.00%	3.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,313,763	$ 9,077,007
Percentage of Book Value	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,164,413	$ 4,039,521
Percentage of Book Value	45.00%	45.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,871,777	$ 1,903,544
Percentage of Book Value	20.00%	21.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 551,701	$ 580,112
Percentage of Book Value	6.00%	6.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 205,800	$ 127,434
Percentage of Book Value	2.00%	1.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 802,471	$ 684,944
Percentage of Book Value	9.00%	8.00%
Manufactured Housing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,349,509	$ 1,361,572
Percentage of Book Value	14.00%	15.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 368,092	$ 379,880
Percentage of Book Value	4.00%	4.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 2,190,170	$ 2,260,392
Percentage of Book Value	23.00%	25.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,802,381	$ 1,836,766
Percentage of Book Value	18.00%	20.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,354,256	$ 1,125,238
Percentage of Book Value	15.00%	12.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 953,204	$ 1,035,839
Percentage of Book Value	10.00%	11.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 703,285	$ 616,964
Percentage of Book Value	8.00%	7.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 350,545	$ 390,307
Percentage of Book Value	4.00%	4.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 7,353,841	$ 7,265,506
Percentage of Book Value	78.00%	79.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 982,404	$ 888,998
Percentage of Book Value	11.00%	10.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 604,343	$ 561,023
Percentage of Book Value	6.00%	6.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 69,180	$ 66,661
Percentage of Book Value	1.00%	1.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 61,795	$ 59,525
Percentage of Book Value	1.00%	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 242,200	$ 235,294
Percentage of Book Value	3.00%	3.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 1,959,922	$ 1,811,501
Percentage of Book Value	22.00%	21.00%