Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 2,022.4	$ 1,998.2
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	0.2	163.8
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	15.8	19.0
Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	14.6	15.5
Level 3 | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	901.3	890.5
Level 3 | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	150.0	150.0
Level 3 | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	35.0	35.0
Level 3 | Discounted cash flow | Preferred Securities
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 29.1	28.2
Level 3 | Probability weighted expected return method | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value		$ 69.1
Level 3 | Discount Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24.00%	24.00%
Level 3 | Discount Rate | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	21.00%	21.00%
Level 3 | Discount Rate | Discounted cash flow | Preferred Securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	8.00%	8.00%
Level 3 | Discount Rate | Discounted cash flow | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	11.00%	11.00%
Level 3 | Discount Rate | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	16.00%	18.00%
Level 3 | Discount Rate | Discounted cash flow | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12.00%	12.00%
Level 3 | Discount Rate | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	24.00%	25.00%
Level 3 | Terminal Cash Flow Exit Multiple | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	700.00%	700.00%
Level 3 | Terminal Cash Flow Exit Multiple | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	2200.00%	2200.00%
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4.00%	4.00%
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	4.00%	4.00%
Level 3 | Measurement Input, Pre-Tax Discount Rate | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	19.00%	20.00%
Level 3 | Measurement, Input, Pre-Tax Terminal Cash Flow Exit Multiple | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	1400.00%	1300.00%