Investments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net unrealized appreciation (depreciation) on investments	Net unrealized appreciation (depreciation) on investments
Level 3
Schedule of Investments [Line Items]
Pre-tax net unrealized appreciation (depreciation)	$ (43.5)	$ 13.8
New Portfolio Investments
Schedule of Investments [Line Items]
Purchases of portfolio investments	694.1	309.6	$ 733.2
Existing Portfolio
Schedule of Investments [Line Items]
Purchases of portfolio investments	648.2	252.2	283.5
Additional Investment In Joint Ventures Investments
Schedule of Investments [Line Items]
Purchases of portfolio investments	1.5	2.5	$ 8.9
New Investment In Joint Ventures Investments
Schedule of Investments [Line Items]
Purchases of portfolio investments		7.9
Equity Co-Investments
Schedule of Investments [Line Items]
Purchases of portfolio investments		108.0
Investments With No Previous Contractual Commitment To Provide Financing
Schedule of Investments [Line Items]
Purchases of portfolio investments	1,231.3	569.5
Investment With Previous Commitment To Provide Financing
Schedule of Investments [Line Items]
Purchases of portfolio investments	$ 112.5	$ 110.7