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Borrowings - Outstanding Debt Obligations (Details) £ in Millions, $ in Millions	Dec. 31, 2023 USD ($)	Dec. 31, 2023 EUR (€)	Dec. 31, 2023 CAD ($)	Dec. 31, 2023 GBP (£)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 EUR (€)	Dec. 31, 2022 CAD ($)	Dec. 31, 2022 GBP (£)	Sep. 30, 2021 USD ($)	Jul. 23, 2021 USD ($)	Apr. 27, 2021 USD ($)	Mar. 16, 2021 USD ($)	Dec. 01, 2020 USD ($)	Oct. 23, 2020 USD ($)	Jul. 15, 2020 USD ($)
Debt Instrument [Line Items]
Aggregate Principal Committed	$ 6,600,000,000				$ 6,550,000,000
Outstanding Principal	4,937,883,000				5,563,003,000
Carrying Value	4,911,930,000				5,527,715,000
Unamortized Debt Issuance Costs	25,953,000				35,289,000
Unused portion	1,662,117,000				986,997,000
Amount Available	1,662,037,000				976,379,000
Jackson Hole Funding Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	500,000,000	€ 0			400,000,000	€ 0
Outstanding Principal	233,019,000				360,019,000
Carrying Value	233,019,000				360,019,000
Unused portion	266,981,000				39,981,000
Amount Available	266,981,000				39,981,000
Breckenridge Funding Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,025,000,000				825,000,000
Outstanding Principal	741,700,000				825,000,000
Carrying Value	741,700,000				825,000,000
Unused portion	283,300,000				0
Amount Available	283,300,000				0
Big Sky Funding Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	500,000,000				500,000,000
Outstanding Principal	480,906,000				499,606,000
Carrying Value	480,906,000				499,606,000
Unused portion	19,094,000				394,000
Amount Available	19,094,000				394,000
Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,775,000,000	€ 94,400,000	$ 1.0	£ 66.9	1,625,000,000	€ 97,900,000	$ 355.9	£ 66.6
Outstanding Principal	682,258,000				678,378,000
Carrying Value	682,258,000				678,378,000
Unused portion	1,092,742,000				946,622,000
Amount Available	1,092,662,000				936,004,000
2023 Notes \| Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed					400,000,000										$ 400,000,000
Outstanding Principal					400,000,000
Carrying Value					398,850,000
Unamortized Debt Issuance Costs					1,150,000
2026 Notes \| Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	800,000,000				800,000,000								$ 300,000,000	$ 500,000,000
Outstanding Principal	800,000,000				800,000,000
Carrying Value	796,343,000				794,559,000
Unamortized Debt Issuance Costs	3,657,000				5,441,000
New 2026 Notes \| Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	700,000,000				700,000,000						$ 300,000,000	$ 400,000,000
Outstanding Principal	700,000,000				700,000,000
Carrying Value	695,206,000				693,432,000
Unamortized Debt Issuance Costs	4,794,000				6,568,000
2027 Notes \| Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	650,000,000				650,000,000					$ 650,000,000
Outstanding Principal	650,000,000				650,000,000
Carrying Value	641,412,000				638,669,000
Unamortized Debt Issuance Costs	8,588,000				11,332,000
2028 Notes \| Unsecured debt
Debt Instrument [Line Items]
Aggregate Principal Committed	650,000,000				650,000,000				$ 650,000,000
Outstanding Principal	650,000,000				650,000,000
Carrying Value	641,086,000				639,202,000
Unamortized Debt Issuance Costs	$ 8,914,000				$ 10,798,000