Commercial Mortgage, Subordinate Loans and Other Lending Assets, Net - Allocation of Carrying Value of Loan Portfolio Based on Internal Risk Ratings (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2021 USD ($) loan	Mar. 31, 2020 USD ($) loan	Dec. 31, 2020 USD ($) loan
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	67	75	67
Total	$ 6,856,509	$ 6,488,891	$ 6,535,079
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	100.00%	100.00%	100.00%
Year 1	$ 410,664	$ 423,419
Year 2	479,012	2,609,209
Year 3	2,661,177	1,545,407
Year 4	1,258,870	905,895
Year 5	1,068,989	216,777
Prior	977,797	788,184
General CECL Allowance	(36,435)	(58,273)	$ (38,102)
Total carrying value, net	$ 6,820,074	$ 6,430,618	$ 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	5
Total	$ 32,000	$ 130,609
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	0.50%	2.00%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	23,990
Year 4	0	0
Year 5	0	36,287
Prior	$ 32,000	$ 70,332
3
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	62	63
Total	$ 6,420,436	$ 5,822,078
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	93.60%	89.70%
Year 1	$ 410,664	$ 423,419
Year 2	479,012	2,609,209
Year 3	2,661,177	1,490,045
Year 4	1,258,870	779,882
Year 5	917,193	62,580
Prior	$ 693,520	$ 456,943
4
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
5
Financing Receivable, Credit Quality Indicator [Line Items]
Number of Loans \| loan	4	7
Total	$ 404,073	$ 536,204
Mortgage loans on real estate, commercial and consumer, percentage of portfolio	5.90%	8.30%
Year 1	$ 0	$ 0
Year 2	0	0
Year 3	0	31,372
Year 4	0	126,013
Year 5	151,796	117,910
Prior	$ 252,277	$ 260,909

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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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