Investment Securities - Significant Unobservable Inputs (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	$ 1,895.8	$ 1,488.0
Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	14.8	9.6
Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
New and existing contracts deployed	108.3	99.8
Level 3 | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	41.7	41.1
Level 3 | Discounted cash flow | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Other long-term investments	14.8	9.6
Level 3 | Discounted cash flow | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	775.3	695.9
Level 3 | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	150.0	135.0
Level 3 | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	35.0	30.0
Level 3 | Discounted cash flow | Mandatorily redeemable preferred securities
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value	$ 58.5
Level 3 | Probability weighted expected return method | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Fair Value		$ 189.0
Level 3 | Discount Rate | Discounted cash flow | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate		11.00%
Level 3 | Discount Rate | Discounted cash flow | PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25.00%	24.00%
Level 3 | Discount Rate | Discounted cash flow | Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	25.00%	21.00%
Level 3 | Discount Rate | Discounted cash flow | Mandatorily redeemable preferred securities
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	15.00%
Level 3 | Discount Rate | Discounted cash flow | Minimum | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	12.00%
Level 3 | Discount Rate | Discounted cash flow | Minimum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	19.00%	18.00%
Level 3 | Discount Rate | Discounted cash flow | Maximum | Private debt instruments
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	13.00%
Level 3 | Discount Rate | Discounted cash flow | Maximum | Kudu’s Participation Contracts
Quantitative information for Level 3 Fair Value Measurements Assets
Discount Rate	26.00%	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%	1600.00%
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	21.00%	21.00%
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	1300.00%	1200.00%