Commercial Mortgage Loans, 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, 2021 USD ($) loan	Dec. 31, 2020 USD ($) loan
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	66	67
Total	$ 7,289,988	$ 6,535,079
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	100.00%	100.00%
Year 1	$ 1,335,670	$ 469,586
Year 2	621,486	2,661,017
Year 3	2,607,658	1,398,479
Year 4	1,037,008	999,564
Year 5	788,435	204,129
Prior	899,731	802,304
General CECL Allowance	(32,684)	(38,102)
Total carrying value, net	$ 7,257,304	$ 6,496,977
Weighted Average Risk Rating	3.1	3.1
1
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	0	0
Total	$ 0	$ 0
Mortgage loans on real estate, commercial and consumer, percentage 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	1
Total	$ 32,000	$ 32,000
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	0.40%	0.50%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	0
Year 4	0	0
Year 5	0	0
Prior	$ 32,000	$ 32,000
3
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	61	62
Total	$ 6,776,238	$ 6,129,541
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	93.00%	93.80%
Year 1	$ 1,335,670	$ 469,586
Year 2	621,486	2,661,017
Year 3	2,607,658	1,398,479
Year 4	1,037,008	868,514
Year 5	531,930	88,710
Prior	$ 642,486	$ 643,235
4
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	1	0
Total	$ 81,980	$ 0
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	1.10%	0.00%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	0
Year 4	0	0
Year 5	81,980	0
Prior	$ 0	$ 0
5
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	3	4
Total	$ 399,770	$ 373,538
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	5.50%	5.70%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	0
Year 4	0	131,050
Year 5	174,525	115,419
Prior	$ 225,245	$ 127,069

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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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