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Borrowings - Outstanding Debt Obligations (Details) £ in Millions, $ in Millions	Mar. 31, 2023 USD ($)	Mar. 31, 2023 CAD ($)	Mar. 31, 2023 EUR (€)	Mar. 31, 2023 GBP (£)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 CAD ($)	Dec. 31, 2022 EUR (€)	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	$ 6,550,000,000				$ 6,550,000,000
Outstanding Principal	5,484,594,000				5,563,003,000
Carrying Value	5,451,854,000				5,527,715,000
Unamortized Debt Issuance Costs	32,740,000				35,289,000
Unused Portion	1,065,406,000				986,997,000
Amount Available	1,065,406,000				976,379,000
Jackson Hole Funding Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	400,000,000				400,000,000		€ 0
Outstanding Principal	360,019,000				360,019,000
Carrying Value	360,019,000				360,019,000
Unused Portion	39,981,000				39,981,000
Amount Available	39,981,000				39,981,000
Breckenridge Funding Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	825,000,000				825,000,000
Outstanding Principal	809,500,000				825,000,000
Carrying Value	809,500,000				825,000,000
Unused Portion	15,500,000				0
Amount Available	15,500,000				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	499,606,000				499,606,000
Carrying Value	499,606,000				499,606,000
Unused Portion	394,000				394,000
Amount Available	394,000				394,000
Revolving Credit Facility \| Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Aggregate Principal Committed	1,625,000,000	$ 274.9	€ 97,900,000	£ 65.4	1,625,000,000	$ 355.9	€ 97,900,000	£ 66.6
Outstanding Principal	615,469,000				678,378,000
Carrying Value	615,469,000				678,378,000
Unused Portion	1,009,531,000				946,622,000
Amount Available	1,009,531,000				936,004,000
3.650% Notes Due 2023 \| Unsecured Debt
Debt Instrument [Line Items]
Aggregate Principal Committed	400,000,000				400,000,000										$ 400,000,000
Outstanding Principal	400,000,000				400,000,000
Carrying Value	399,381,000				398,850,000
Unamortized Debt Issuance Costs	619,000				1,150,000
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	795,001,000				794,559,000
Unamortized Debt Issuance Costs	4,999,000				5,441,000
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	693,868,000				693,432,000
Unamortized Debt Issuance Costs	6,132,000				6,568,000
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	639,346,000				638,669,000
Unamortized Debt Issuance Costs	10,654,000				11,332,000
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	639,664,000				639,202,000
Unamortized Debt Issuance Costs	$ 10,336,000				$ 10,798,000