Commercial Mortgage, Subordinate Loans and Other Lending Assets, Net - Allocation of Carrying Value of Loan Portfolio Based on Internal Risk Ratings (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	70	72
Total	$ 6,476,313	$ 6,375,093
% of Portfolio	100.00%	100.00%
Year 1	$ 450,119	$ 2,736,825
Year 2	2,685,060	1,596,690
Year 3	1,379,821	912,636
Year 4	953,587	291,700
Year 5	203,595	524,246
Prior	804,131	312,996
General CECL Allowance	(39,276)
Total carrying value, net	$ 6,437,037	$ 6,375,093
Weighted Average Risk Rating	3.1	3.0
1
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	0	0
Total	$ 0	$ 0
% of Portfolio	0.00%	0.00%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	0
Year 4	0	0
Year 5	0	0
Prior	$ 0	$ 0
2
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	1	8
Total	$ 32,000	$ 348,324
% of Portfolio	0.50%	5.50%
Year 1	$ 0	$ 0
Year 2	0	241,676
Year 3	0	0
Year 4	0	36,250
Year 5	0	24,546
Prior	$ 32,000	$ 45,852
3
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	62	61
Total	$ 6,035,647	$ 5,707,555
% of Portfolio	93.20%	89.50%
Year 1	$ 450,119	$ 2,736,825
Year 2	2,685,060	1,355,014
Year 3	1,350,949	912,636
Year 4	815,789	72,540
Year 5	88,685	499,700
Prior	$ 645,045	$ 130,840
4
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	0	1
Total	$ 0	$ 182,910
% of Portfolio	0.00%	2.90%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	0
Year 4	0	182,910
Year 5	0	0
Prior	$ 0	$ 0
5
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	7	2
Total	$ 408,666	$ 136,304
% of Portfolio	6.30%	2.10%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	28,872	0
Year 4	137,798	0
Year 5	114,910	0
Prior	$ 127,086	$ 136,304

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The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio.

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