Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 1,998.2	$ 1,488.0
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	19.0	41.1
Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 15.5	$ 9.6
Level 3 | Discounted cash flow | Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate		11.00%
Level 3 | Discounted cash flow | Discount Rate | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	11.00%
Level 3 | Discounted cash flow | Discount Rate | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12.00%
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	$ 163.8	$ 99.8
Kudu’s Participation Contracts | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 890.5	$ 695.9
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Discount Rate | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	18.00%	18.00%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Discount Rate | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25.00%	25.00%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Terminal Cash Flow Exit Multiple | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	700.00%	700.00%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Terminal Cash Flow Exit Multiple | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	2200.00%	1600.00%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Measurement Input, Pre-Tax Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	20.00%	21.00%
Kudu’s Participation Contracts | Level 3 | Discounted cash flow | Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	1300.00%	1200.00%
Kudu’s Participation Contracts | Level 3 | Probability weighted expected return method
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 69.1	$ 189.0
PassportCard/DavidShield | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 150.0	$ 135.0
PassportCard/DavidShield | Level 3 | Discounted cash flow | Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24.00%	24.00%
PassportCard/DavidShield | Level 3 | Discounted cash flow | Terminal Revenue Growth Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4.00%	4.00%
Elementum Holdings, L.P. | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 35.0	$ 30.0
Elementum Holdings, L.P. | Level 3 | Discounted cash flow | Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	21.00%	21.00%
Elementum Holdings, L.P. | Level 3 | Discounted cash flow | Terminal Revenue Growth Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4.00%	4.00%
Preferred securities | Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 28.2
Preferred securities | Level 3 | Discounted cash flow | Discount Rate
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	8.00%