Investment Securities - Significant Unobservable Inputs (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 2,589.2	$ 2,150.2
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	119.3	103.5
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	4.9	23.4
Level 3 | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	15.6	14.5
Level 3 | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	1,208.2	1,008.4
Level 3 | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	160.0	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	33.6	$ 31.4
Level 3 | Discounted cash flow | Private equity securities
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 21.9
Level 3 | Discount Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	0.24	0.24
Level 3 | Discount Rate | Discounted cash flow | Elementum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	0.22	0.22
Level 3 | Discount Rate | Discounted cash flow | Preferred securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	0.08	0.09
Level 3 | Discount Rate | Discounted cash flow | Private equity securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	0.35
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	0.16	0.17
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	0.25	0.25
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	7	7
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	22	22
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	0.04	0.04
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | Elementum
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	0.04	0.04
Level 3 | Terminal Revenue Growth Rate | Discounted cash flow | Private equity securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	0.04
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	0.19	0.19
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	14	14