Investment Securities - Significant Unobservable Inputs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in trading securities	$ 178.9	$ 118.3	$ 118.4
Kudu
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in trading securities		118.2
PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in trading securities		15.0
Other long-term investments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	46.7	$ 27.6
TIG Advisors LLC, TK Partners LLC, Third Eye Capital Management And Douglass Winthrop Advisors | Kudu
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in trading securities	$ 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	0.20	0.20
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)	23.00%	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)	23.00%	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)	17.00%	17.00%
Measurement Input, Discount Rate | Private debt investments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	8.00%
Measurement Input, Discount Rate | Private debt investments | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)		4.00%
Measurement Input, Discount Rate | Private debt investments | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)		8.00%
Measurement Input, Discount Rate | Other long-term investments | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)		20.00%
Measurement Input, Discount Rate | Other long-term investments | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)		24.00%
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%
Measurement Input, Long-term Revenue Growth Rate | Other long-term investments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)		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	10	10
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	13	12
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	$ 669.5	$ 400.6
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	120.0	95.0
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	45.0	55.1
Discounted Cash Flow Valuation Technique | Private debt investments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	9.4	17.1
Discounted Cash Flow Valuation Technique | Other long-term investments | Standard Poors NR Rating | Level 3
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 18.8
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	$ 78.8