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Borrowings - Outstanding Debt Obligations (Details) £ in Millions, $ in Millions, $ in Millions	Mar. 31, 2024 USD ($)	Mar. 31, 2024 EUR (€)	Mar. 31, 2024 CAD ($)	Mar. 31, 2024 GBP (£)	Mar. 31, 2024 AUD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2023 EUR (€)	Dec. 31, 2023 CAD ($)	Dec. 31, 2023 GBP (£)	Sep. 30, 2021 USD ($)	Jul. 23, 2021 USD ($)	Apr. 27, 2021 USD ($)	Mar. 16, 2021 USD ($)	Dec. 01, 2020 USD ($)	Oct. 23, 2020 USD ($)
Debt Instrument [Line Items]
Aggregate Principal Committed	$ 6,600,000,000					$ 6,600,000,000
Outstanding Principal	5,298,027,000					4,937,883,000
Carrying Value	5,274,114,000					4,911,930,000
Unamortized Debt Issuance Costs	23,913,000					25,953,000
Unused portion	1,301,974,000					1,662,117,000
Amount available	1,301,974,000					1,662,037,000
Jackson Hole Funding Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	500,000,000	€ 0				500,000,000	€ 0
Outstanding Principal	387,949,000					233,019,000
Carrying Value	387,949,000					233,019,000
Unused portion	112,051,000					266,981,000
Amount available	112,051,000					266,981,000
Breckenridge Funding Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,025,000,000					1,025,000,000
Outstanding Principal	824,350,000					741,700,000
Carrying Value	824,350,000					741,700,000
Unused portion	200,650,000					283,300,000
Amount available	200,650,000					283,300,000
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	500,000,000					480,906,000
Carrying Value	500,000,000					480,906,000
Unused portion	0					19,094,000
Amount available	0					19,094,000
Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,775,000,000	€ 101,100,000	$ 1.0	£ 113.2	$ 1.0	1,775,000,000	€ 94,400,000	$ 1.0	£ 66.9
Outstanding Principal	785,728,000					682,258,000
Carrying Value	785,728,000					682,258,000
Unused portion	989,273,000					1,092,742,000
Amount available	989,273,000					1,092,662,000
3.625% Notes Due 2026 \| 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	796,789,000					796,343,000
Unamortized Debt Issuance Costs	3,211,000					3,657,000
2.750% Notes Due 2026 \| 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	695,647,000					695,206,000
Unamortized Debt Issuance Costs	4,353,000					4,794,000
2.125% Senior Notes Due 2027 \| 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	642,097,000					641,412,000
Unamortized Debt Issuance Costs	7,903,000					8,588,000
2.850% Notes Due 2028 \| 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	641,554,000					641,086,000
Unamortized Debt Issuance Costs	$ 8,446,000					$ 8,914,000