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Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2025	Dec. 31, 2024
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 2,287.9	$ 2,150.2
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	68.0	103.5
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	23.6	23.4
Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	14.9	14.5
Level 3 | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	1,120.4	1,008.4
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
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	$ 32.0	$ 31.4
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
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	22.00%	22.00%
Level 3 | Discount Rate | Discounted cash flow | Preferred securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	9.00%	9.00%
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%	17.00%
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
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%	19.00%
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%	1400.00%

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