Fair Value of Financial Instruments - Unobservable Inputs (Details) $ in Millions	Jun. 30, 2023 USD ($)		Dec. 31, 2022 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 14,764.0	[1]	$ 15,377.3	[2]
Subordinated Debt
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	288.1	[1]	265.0	[2]
Asset Based Finance
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,779.5	[1]	1,902.5	[2]
Equity/Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	1,235.0		1,199.0
Level 3—Significant unobservable inputs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	12,839.0		13,385.0
Level 3—Significant unobservable inputs | Senior Debt | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 8,596.0		$ 9,274.0
Level 3—Significant unobservable inputs | Senior Debt | Discounted Cash Flow | Discount Rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.063		0.060
Level 3—Significant unobservable inputs | Senior Debt | Discounted Cash Flow | Discount Rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.200		0.242
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	0.117		0.120
Level 3—Significant unobservable inputs | Senior Debt | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 753.0		$ 575.0
Level 3—Significant unobservable inputs | Senior Debt | Waterfall | EBITDA Multiple | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.2		0.3
Level 3—Significant unobservable inputs | Senior Debt | Waterfall | EBITDA Multiple | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	14.5		19.8
Level 3—Significant unobservable inputs | Senior Debt | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7		7.2
Level 3—Significant unobservable inputs | Senior Debt | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 154.0		$ 4.0
Level 3—Significant unobservable inputs | Senior Debt | Cost
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	36.0		167.0
Level 3—Significant unobservable inputs | Subordinated Debt | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 256.0		$ 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		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		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	0.130		0.133
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 31.0		$ 22.0
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall | EBITDA Multiple | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	6.9		7.5
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall | EBITDA Multiple | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	6.9		7.5
Level 3—Significant unobservable inputs | Subordinated Debt | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	6.9		7.5
Level 3—Significant unobservable inputs | Asset Based Finance | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 926.0		$ 980.0
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	0.054		0.051
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	0.450		0.440
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	0.114		0.113
Level 3—Significant unobservable inputs | Asset Based Finance | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 552.0		$ 636.0
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		13.7
Level 3—Significant unobservable inputs | Asset Based Finance | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.2		1.6
Level 3—Significant unobservable inputs | Asset Based Finance | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 142.0		$ 138.0
Level 3—Significant unobservable inputs | Asset Based Finance | Cost
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	155.0		144.0
Level 3—Significant unobservable inputs | Asset Based Finance | Indicative Dealer Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
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	0.440		0.420
Level 3—Significant unobservable inputs | Asset Based Finance | Indicative Dealer Quotes | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.440		0.420
Level 3—Significant unobservable inputs | Asset Based Finance | Indicative Dealer Quotes | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.440		0.420
Level 3—Significant unobservable inputs | Equity/Other | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 499.0		$ 488.0
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		0.100
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	0.157		0.156
Level 3—Significant unobservable inputs | Equity/Other | Waterfall
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 728.0		$ 683.0
Level 3—Significant unobservable inputs | Equity/Other | Waterfall | EBITDA Multiple | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.6		0.0
Level 3—Significant unobservable inputs | Equity/Other | Waterfall | EBITDA Multiple | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	16.0		15.0
Level 3—Significant unobservable inputs | Equity/Other | Waterfall | EBITDA Multiple | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	7.2		7.4
Level 3—Significant unobservable inputs | Equity/Other | Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 6.0		$ 12.0
Level 3—Significant unobservable inputs | Equity/Other | Cost
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value			13.0
Level 3—Significant unobservable inputs | Equity/Other | Option Pricing Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 1.0		$ 3.0
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] See Note 8 for additional information regarding the fair value of the Company's financial instruments

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