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, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	122	125
Net book value	$ 14,508,735	$ 14,191,200
Total Loan Exposure	$ 15,053,372	$ 14,740,844
Percentage of Portfolio	100.00%	100.00%
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	52	55
Net book value	$ 7,080,970	$ 7,104,842
Total Loan Exposure	$ 7,133,812	$ 7,164,466
Percentage of Portfolio	47.00%	49.00%
Hotel [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	18	18
Net book value	$ 2,612,768	$ 2,591,565
Total Loan Exposure	$ 2,694,028	$ 2,673,763
Percentage of Portfolio	18.00%	18.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	35	34
Net book value	$ 2,238,435	$ 2,193,699
Total Loan Exposure	$ 2,256,289	$ 2,206,740
Percentage of Portfolio	15.00%	15.00%
Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	5
Net book value	$ 681,091	$ 680,808
Total Loan Exposure	$ 685,513	$ 685,776
Percentage of Portfolio	5.00%	5.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	4
Net book value	$ 457,968	$ 451,099
Total Loan Exposure	$ 460,423	$ 452,900
Percentage of Portfolio	3.00%	3.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	4
Net book value	$ 278,110	$ 304,545
Total Loan Exposure	$ 328,500	$ 368,104
Percentage of Portfolio	2.00%	2.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	3
Net book value	$ 876,709	$ 586,169
Total Loan Exposure	$ 1,210,851	$ 909,052
Percentage of Portfolio	8.00%	6.00%
Self-Storage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 282,684	$ 278,473
Total Loan Exposure	$ 283,956	$ 280,043
Percentage of Portfolio	2.00%	2.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	33	32
Net book value	$ 4,481,059	$ 4,322,114
Total Loan Exposure	$ 4,512,296	$ 4,359,938
Percentage of Portfolio	32.00%	31.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	26	29
Net book value	$ 2,996,086	$ 3,137,072
Total Loan Exposure	$ 3,072,901	$ 3,222,706
Percentage of Portfolio	20.00%	22.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	18	19
Net book value	$ 2,249,064	$ 2,258,033
Total Loan Exposure	$ 2,261,069	$ 2,271,664
Percentage of Portfolio	15.00%	15.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	9
Net book value	$ 1,170,535	$ 1,161,637
Total Loan Exposure	$ 1,178,563	$ 1,170,619
Percentage of Portfolio	8.00%	8.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	13	13
Net book value	$ 479,835	$ 478,665
Total Loan Exposure	$ 482,734	$ 481,745
Percentage of Portfolio	3.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	4
Net book value	$ 156,639	$ 238,844
Total Loan Exposure	$ 157,462	$ 239,872
Percentage of Portfolio	1.00%	2.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	102	106
Net book value	$ 11,533,218	$ 11,596,365
Total Loan Exposure	$ 11,665,025	$ 11,746,544
Percentage of Portfolio	79.00%	81.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	7
Net book value	$ 1,154,538	$ 754,299
Total Loan Exposure	$ 1,507,808	$ 1,094,663
Percentage of Portfolio	10.00%	7.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 1,069,279	$ 1,124,174
Total Loan Exposure	$ 1,075,640	$ 1,131,334
Percentage of Portfolio	7.00%	8.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	5	5
Net book value	$ 341,106	$ 316,268
Total Loan Exposure	$ 338,462	$ 313,229
Percentage of Portfolio	2.00%	2.00%
Australia [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	3
Net book value	$ 329,780	$ 310,372
Total Loan Exposure	$ 332,107	$ 312,893
Percentage of Portfolio	2.00%	2.00%
Belgium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 69,057	$ 70,621
Total Loan Exposure	$ 69,378	$ 71,007
Percentage of Portfolio	8212.00%	8212.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 11,757	$ 11,585
Total Loan Exposure	$ 64,952	$ 63,637
Percentage of Portfolio	8212.00%	8212.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan		1
Net book value		$ 7,516
Total Loan Exposure		$ 7,537
Percentage of Portfolio		8212.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	20	19
Net book value	$ 2,975,517	$ 2,594,835
Total Loan Exposure	$ 3,388,347	$ 2,994,300
Percentage of Portfolio	21.00%	19.00%