Secured Debt Arrangements, Net - Summary of Outstanding Balances, Maximum and Average Balances of Borrowings (Details) - Line of Credit - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Balance	$ 3,450,858	$ 3,449,665
Amortized Cost of Collateral	5,593,505	5,418,999
JPMorgan
Line of Credit Facility [Line Items]
Balance	1,068,184	1,170,969
Amortized Cost of Collateral	2,111,256	2,009,249
Maximum Month-End Balance	1,359,053	1,192,288
Average Month-End Balance	1,177,128	1,119,997
DB
Line of Credit Facility [Line Items]
Balance	398,452	520,457
Amortized Cost of Collateral	617,745	814,715
Maximum Month-End Balance	520,217	526,743
Average Month-End Balance	443,190	506,831
Goldman
Line of Credit Facility [Line Items]
Balance	142,771	332,352
Amortized Cost of Collateral	288,320	510,371
Maximum Month-End Balance	331,154	362,139
Average Month-End Balance	241,145	343,621
CS Facility | USD
Line of Credit Facility [Line Items]
Balance	189,220	369,182
Amortized Cost of Collateral	277,828	524,139
Maximum Month-End Balance	369,182	378,781
Average Month-End Balance	240,562	348,464
CS Facility | Fx contracts - GBP
Line of Credit Facility [Line Items]
Balance		0
Amortized Cost of Collateral		0
Maximum Month-End Balance		90,111
Average Month-End Balance		43,094
HSBC Facility | USD
Line of Credit Facility [Line Items]
Balance		0
Amortized Cost of Collateral		0
Maximum Month-End Balance		50,625
Average Month-End Balance		44,000
HSBC Facility | Fx contracts - GBP
Line of Credit Facility [Line Items]
Balance		0
Amortized Cost of Collateral		0
Maximum Month-End Balance		34,500
Average Month-End Balance		20,563
HSBC Facility | Fx contracts - EUR
Line of Credit Facility [Line Items]
Balance	165,947	163,785
Amortized Cost of Collateral	209,407	215,509
Maximum Month-End Balance	174,717	163,788
Average Month-End Balance	166,717	154,725
Barclays Facility | USD
Line of Credit Facility [Line Items]
Balance	32,693	35,192
Amortized Cost of Collateral	50,186	49,993
Maximum Month-End Balance	35,193	35,193
Average Month-End Balance	33,804	29,327
Barclays Facility | Fx contracts - GBP
Line of Credit Facility [Line Items]
Balance		0
Amortized Cost of Collateral		0
Maximum Month-End Balance		666,810
Average Month-End Balance		260,692
Barclays Facility | Fx contracts - EUR
Line of Credit Facility [Line Items]
Balance		0
Amortized Cost of Collateral		0
Maximum Month-End Balance		180,595
Average Month-End Balance		70,521
Barclays Securitization
Line of Credit Facility [Line Items]
Balance	1,453,591	857,728
Amortized Cost of Collateral	2,038,763	1,295,023
Maximum Month-End Balance	1,499,314	857,728
Average Month-End Balance	$ 1,286,587	$ 823,915