Investment Securities - Significant Unobservable Inputs (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2022 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in trading securities	$ 0.0	$ 11.0
Kudu investments | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, pretax discount rate	0.20		0.20
Cash flow exit multiple, pretax	10		10
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
Level 3 | Other long-term investments | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 14.4		46.7
Level 3 | Kudu investments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	691.8		669.5
Level 3 | Kudu investments | Standard Poors NR Rating | Probability weighted expected return method
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 81.9		$ 78.8
Level 3 | Kudu investments | Discount rate | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%		18.00%
Level 3 | Kudu investments | Discount rate | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%		23.00%
Level 3 | Kudu investments | Terminal Multiple | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal cash flow exit multiple	7		7
Level 3 | Kudu investments | Terminal Multiple | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal cash flow exit multiple	13		13
Level 3 | PassportCard/DavidShield | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 125.0		$ 120.0
Level 3 | PassportCard/DavidShield | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%		23.00%
Level 3 | PassportCard/DavidShield | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate	4.00%		4.00%
Level 3 | Elementum Holdings, L.P. | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 45.0		$ 45.0
Level 3 | Elementum Holdings, L.P. | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	17.00%		17.00%
Level 3 | Elementum Holdings, L.P. | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate	4.00%		4.00%
Level 3 | Private debt investments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 9.5		$ 9.4
Level 3 | Private debt investments | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	9.00%		8.00%