Investment Securities - Significant Unobservable Inputs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Other long-term investments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 41.1	$ 46.7
Kudu Participation Contracts | Kudu
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in trading securities	$ 99.8	$ 223.4
Measurement Input, Discount Rate | Kudu's Participation Contracts | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, pretax discount rate	21.00%	20.00%
Measurement Input, Discount Rate | Kudu's Participation Contracts | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	18.00%	18.00%
Measurement Input, Discount Rate | Kudu's Participation Contracts | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	25.00%	23.00%
Measurement Input, Discount Rate | PassportCard/DavidShield | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	24.00%	23.00%
Measurement Input, Discount Rate | Elementum Holdings, L.P. | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	21.00%	17.00%
Measurement Input, Discount Rate | Private debt instruments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	11.00%	8.00%
Measurement Input, Long-term Revenue Growth Rate | PassportCard/DavidShield | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)	4.00%	4.00%
Measurement Input, Long-term Revenue Growth Rate | Elementum Holdings, L.P. | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)	4.00%	4.00%
Terminal Multiple | Kudu's Participation Contracts | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Cash flow exit multiple, pretax	11.8	10.0
Terminal Multiple | Kudu's Participation Contracts | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Cash flow exit multiple	7	7
Terminal Multiple | Kudu's Participation Contracts | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Cash flow exit multiple	16	13
Discounted Cash Flow Valuation Technique | Kudu's Participation Contracts | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 695.9	$ 669.5
Discounted Cash Flow Valuation Technique | PassportCard/DavidShield | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	135.0	120.0
Discounted Cash Flow Valuation Technique | Elementum Holdings, L.P. | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	30.0	45.0
Discounted Cash Flow Valuation Technique | Private debt instruments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	9.6	9.4
Probability Weighted Expected Return Method | Kudu's Participation Contracts | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 189.0	$ 78.8