Fair Value of Financial Instruments - Unobservable Inputs (Details) $ in Millions	Dec. 31, 2022 USD ($)		Dec. 31, 2021 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 15,377.3	[1]	$ 16,101.5	[2]
Level 3—Significant unobservable inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	13,385.0		13,972.0
Level 3—Significant unobservable inputs | Senior Debt | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 9,274.0		$ 8,746.0
Level 3—Significant unobservable inputs | Senior Debt | Discounted Cash Flow | Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.060		0.053
Level 3—Significant unobservable inputs | Senior Debt | Discounted Cash Flow | Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.242		0.303
Level 3—Significant unobservable inputs | Senior Debt | Discounted Cash Flow | Discount Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.120		0.085
Level 3—Significant unobservable inputs | Senior Debt | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 575.0		$ 737.0
Level 3—Significant unobservable inputs | Senior Debt | Waterfall | EBITDA Multiple | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.3		0.1
Level 3—Significant unobservable inputs | Senior Debt | Waterfall | EBITDA Multiple | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	19.8		11.0
Level 3—Significant unobservable inputs | Senior Debt | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.2		7.0
Level 3—Significant unobservable inputs | Senior Debt | Cost
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 167.0		$ 1,242.0
Level 3—Significant unobservable inputs | Senior Debt | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	4.0		51.0
Level 3—Significant unobservable inputs | Subordinated Debt | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 242.0
Level 3—Significant unobservable inputs | Subordinated Debt | Discounted Cash Flow | Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.109
Level 3—Significant unobservable inputs | Subordinated Debt | Discounted Cash Flow | Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.149
Level 3—Significant unobservable inputs | Subordinated Debt | Discounted Cash Flow | Discount Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.133
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 22.0		$ 53.0
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall | EBITDA Multiple | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.5		7.0
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall | EBITDA Multiple | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.5		7.8
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.5		7.8
Level 3—Significant unobservable inputs | Subordinated Debt | Cost
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value			$ 21.0
Level 3—Significant unobservable inputs | Asset Based Finance | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 980.0		$ 744.0
Level 3—Significant unobservable inputs | Asset Based Finance | Discounted Cash Flow | Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.051		0.042
Level 3—Significant unobservable inputs | Asset Based Finance | Discounted Cash Flow | Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.440		0.162
Level 3—Significant unobservable inputs | Asset Based Finance | Discounted Cash Flow | Discount Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.113		0.101
Level 3—Significant unobservable inputs | Asset Based Finance | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 636.0		$ 1,021.0
Level 3—Significant unobservable inputs | Asset Based Finance | Waterfall | EBITDA Multiple | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0		1.0
Level 3—Significant unobservable inputs | Asset Based Finance | Waterfall | EBITDA Multiple | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	13.7		23.1
Level 3—Significant unobservable inputs | Asset Based Finance | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.6		4.1
Level 3—Significant unobservable inputs | Asset Based Finance | Cost
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 144.0		$ 359.0
Level 3—Significant unobservable inputs | Asset Based Finance | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	138.0		117.0
Level 3—Significant unobservable inputs | Asset Based Finance | Indicative Dealer Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 4.0		$ 4.0
Level 3—Significant unobservable inputs | Asset Based Finance | Indicative Dealer Quotes | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.420		0.508
Level 3—Significant unobservable inputs | Asset Based Finance | Indicative Dealer Quotes | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.420		0.508
Level 3—Significant unobservable inputs | Asset Based Finance | Indicative Dealer Quotes | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.420		0.508
Level 3—Significant unobservable inputs | Equity/Other | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 488.0		$ 111.0
Level 3—Significant unobservable inputs | Equity/Other | Discounted Cash Flow | Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.100		0.073
Level 3—Significant unobservable inputs | Equity/Other | Discounted Cash Flow | Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.250		0.250
Level 3—Significant unobservable inputs | Equity/Other | Discounted Cash Flow | Discount Rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.156		0.098
Level 3—Significant unobservable inputs | Equity/Other | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 683.0		$ 737.0
Level 3—Significant unobservable inputs | Equity/Other | Waterfall | EBITDA Multiple | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0		0.1
Level 3—Significant unobservable inputs | Equity/Other | Waterfall | EBITDA Multiple | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	15.0		16.0
Level 3—Significant unobservable inputs | Equity/Other | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.4		6.1
Level 3—Significant unobservable inputs | Equity/Other | Cost
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 13.0		$ 2.0
Level 3—Significant unobservable inputs | Equity/Other | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	12.0		22.0
Level 3—Significant unobservable inputs | Equity/Other | Option Pricing Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, at fair value	$ 3.0		$ 5.0
Level 3—Significant unobservable inputs | Equity/Other | Option Pricing Model | Equity Illiquidity Discount | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.650		0.650
Level 3—Significant unobservable inputs | Equity/Other | Option Pricing Model | Equity Illiquidity Discount | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.650		0.650
Level 3—Significant unobservable inputs | Equity/Other | Option Pricing Model | Equity Illiquidity Discount | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.650		0.650

[1] Fair value determined by the Company’s board of directors (see Note 8).

[2] Fair value determined by the Company’s board of directors (see Note 8).