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, 2020 USD ($) SecurityLoan	Dec. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	132	128
Net book value	$ 16,363,608	$ 16,164,801
Total Loan Exposure	$ 17,161,035	$ 16,965,864	$ 16,164,801
Percentage of Portfolio	100.00%	100.00%
CECL reserve	$ (112,694)	$ (112,694)
Net book value of restructured loan	$ 16,250,914
Office [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	60	63
Net book value	$ 9,481,796	$ 9,946,055
Total Loan Exposure	$ 9,814,730	$ 10,266,567
Percentage of Portfolio	57.00%	61.00%
Hospitality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	14	14
Net book value	$ 2,194,116	$ 2,199,220
Total Loan Exposure	$ 2,271,536	$ 2,281,718
Percentage of Portfolio	13.00%	13.00%
Multifamily [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	39	36
Net book value	$ 1,957,101	$ 1,596,333
Total Loan Exposure	$ 2,010,565	$ 1,642,664
Percentage of Portfolio	12.00%	10.00%
Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	7	5
Net book value	$ 826,694	$ 603,917
Total Loan Exposure	$ 832,082	$ 607,423
Percentage of Portfolio	5.00%	4.00%
Retail [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	3
Net book value	$ 520,279	$ 373,045
Total Loan Exposure	$ 529,997	$ 381,040
Percentage of Portfolio	3.00%	2.00%
Condominium [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	1
Net book value	$ 228,912	$ 232,778
Total Loan Exposure	$ 230,466	$ 234,260
Percentage of Portfolio	1.00%	1.00%
Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	4	4
Net book value	$ 869,239	$ 921,459
Total Loan Exposure	$ 1,185,871	$ 1,259,696
Percentage of Portfolio	7.00%	7.00%
Self-Storage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 285,471	$ 291,994
Total Loan Exposure	$ 285,788	$ 292,496
Percentage of Portfolio	2.00%	2.00%
United States Northeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	27	25
Net book value	$ 4,222,778	$ 3,789,477
Total Loan Exposure	$ 4,249,684	$ 3,815,580
Percentage of Portfolio	25.00%	22.00%
United States West [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	28	30
Net book value	$ 2,853,616	$ 3,143,323
Total Loan Exposure	$ 3,188,319	$ 3,451,914
Percentage of Portfolio	19.00%	20.00%
United States Southeast [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	26	23
Net book value	$ 2,481,003	$ 2,321,444
Total Loan Exposure	$ 2,493,980	$ 2,334,852
Percentage of Portfolio	15.00%	14.00%
United States Midwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	9	10
Net book value	$ 1,014,714	$ 1,174,581
Total Loan Exposure	$ 1,019,402	$ 1,180,240
Percentage of Portfolio	6.00%	7.00%
United States Southwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	12	11
Net book value	$ 648,949	$ 464,989
Total Loan Exposure	$ 651,714	$ 467,532
Percentage of Portfolio	4.00%	3.00%
United States Northwest [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	3
Net book value	$ 53,124	$ 52,891
Total Loan Exposure	$ 53,187	$ 52,989
Percentage of Portfolio	0.00%
United States [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	105	102
Net book value	$ 11,274,184	$ 10,946,705
Total Loan Exposure	$ 11,656,286	$ 11,303,107
Percentage of Portfolio	69.00%	66.00%
United Kingdom [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	13	13
Net book value	$ 1,631,711	$ 1,738,536
Total Loan Exposure	$ 1,971,765	$ 2,102,501
Percentage of Portfolio	11.00%	12.00%
Spain [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	2	2
Net book value	$ 1,182,830	$ 1,231,061
Total Loan Exposure	$ 1,188,897	$ 1,237,809
Percentage of Portfolio	7.00%	7.00%
Canada [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	3
Net book value	$ 72,103	$ 77,656
Total Loan Exposure	$ 72,313	$ 77,953
Percentage of Portfolio	0.00%	1.00%
Australia [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	3	3
Net book value	$ 319,160	$ 360,047
Total Loan Exposure	$ 320,514	$ 361,763
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	$ 85,551	$ 86,807
Total Loan Exposure	$ 85,786	$ 87,201
Percentage of Portfolio	0.00%	1.00%
Germany [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 195,109	$ 195,081
Total Loan Exposure	$ 247,447	$ 251,020
Percentage of Portfolio	1.00%	1.00%
Netherlands [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1
Net book value	$ 94,791
Total Loan Exposure	$ 95,965
Percentage of Portfolio	1.00%
International [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	27	26
Net book value	$ 5,089,424	$ 5,218,096
Total Loan Exposure	$ 5,504,749	$ 5,662,757
Percentage of Portfolio	31.00%	34.00%
Ireland [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 1,298,062	$ 1,318,196
Total Loan Exposure	$ 1,309,049	$ 1,330,647
Percentage of Portfolio	8.00%	8.00%
France [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 31,501	$ 31,972
Total Loan Exposure	$ 32,431	$ 32,966
Percentage of Portfolio	0.00%	1.00%
Italy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Net book value	$ 178,606	$ 178,740
Total Loan Exposure	$ 180,582	$ 180,897
Percentage of Portfolio	1.00%	1.00%