Summary of Secured Credit Facilities Secured by Commercial Mortgage Loans, CRE Debt Securities and Counterparty Concentration Risks (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 3,144,276	$ 3,239,506
UPB of Collateral	1,402,361	2,262,311
Amortized Cost of Collateral	1,402,562	2,261,912
Amounts Payable under Secured Revolving Repurchase Agreements	863,751	1,524,175
Net Counterparty Exposure	$ 538,811	$ 737,737
Days to Extended Maturity	839 days	938 days
Goldman Sachs Bank | Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 250,000	$ 250,000
UPB of Collateral	160,959	96,381
Amortized Cost of Collateral	162,403	96,843
Amounts Payable under Secured Revolving Repurchase Agreements	98,021	50,909
Net Counterparty Exposure	$ 64,382	$ 45,934
Percent of Stockholders' Equity	5.00%	3.60%
Days to Extended Maturity	415 days	596 days
Wells Fargo | Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 750,000	$ 750,000
UPB of Collateral	169,396	290,237
Amortized Cost of Collateral	169,522	290,403
Amounts Payable under Secured Revolving Repurchase Agreements	63,295	216,734
Net Counterparty Exposure	$ 106,227	$ 73,669
Percent of Stockholders' Equity	8.30%	5.80%
Days to Extended Maturity	292 days	473 days
Barclays | Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 750,000	$ 750,000
UPB of Collateral	229,722	443,845
Amortized Cost of Collateral	229,633	443,620
Amounts Payable under Secured Revolving Repurchase Agreements	150,497	316,524
Net Counterparty Exposure	$ 79,136	$ 127,096
Percent of Stockholders' Equity	6.20%	10.00%
Days to Extended Maturity	409 days	590 days
Morgan Stanley Bank | Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 500,000	$ 500,000
UPB of Collateral	223,976	434,630
Amortized Cost of Collateral	223,790	433,948
Amounts Payable under Secured Revolving Repurchase Agreements	169,157	326,199
Net Counterparty Exposure	$ 54,633	$ 107,749
Percent of Stockholders' Equity	4.30%	8.50%
Days to Extended Maturity	308 days	489 days
JP Morgan Chase Bank | Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 649,546	$ 649,546
UPB of Collateral	516,976	778,453
Amortized Cost of Collateral	515,284	777,862
Amounts Payable under Secured Revolving Repurchase Agreements	311,868	457,041
Net Counterparty Exposure	$ 203,416	$ 320,821
Percent of Stockholders' Equity	15.90%	25.30%
Days to Extended Maturity	1583 days	1764 days
US Bank | Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 44,730	$ 139,960
UPB of Collateral	58,519	101,372
Amortized Cost of Collateral	58,885	101,599
Amounts Payable under Secured Revolving Repurchase Agreements	39,300	69,649
Net Counterparty Exposure	$ 19,585	$ 31,950
Percent of Stockholders' Equity	1.50%	2.50%
Days to Extended Maturity	1105 days	1286 days
Bank of America | Commercial Mortgage Loans
Repurchase Agreement Counterparty [Line Items]
Commitment Amount	$ 200,000	$ 200,000
UPB of Collateral	42,813	117,393
Amortized Cost of Collateral	43,045	117,637
Amounts Payable under Secured Revolving Repurchase Agreements	31,613	87,119
Net Counterparty Exposure	$ 11,432	$ 30,518
Percent of Stockholders' Equity	0.90%	2.40%
Days to Extended Maturity	456 days	637 days