Investment Securities - Significant Unobservable Inputs (Details) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Other Long-Term Investments	$ 141.6	$ 57.5
Kudu
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Other Long-Term Investments			$ 118.2
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%
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%
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
Kudu investments | Terminal Multiple | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal Cash Flow Exit Multiple	13		12
PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Other Long-Term Investments			$ 15.0
Private debt investments | Discount rate | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	4.00%		4.00%
Private debt investments | Discount rate | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	8.00%		8.00%
Other long-term investments | Discount rate | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	20.00%		20.00%
Other long-term investments | Discount rate | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	24.00%		24.00%
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 Other Long-Term Investments	$ 141.6
Level 3 | Kudu investments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	604.7		$ 400.6
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	121.0
Level 3 | PassportCard/DavidShield | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 105.0		$ 95.0
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	$ 56.7		$ 55.1
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	$ 11.2		$ 17.1
Level 3 | Other long-term investments | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	14.9		27.6
Level 3 | Other long-term investments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 22.0		$ 18.8
Level 3 | Other long-term investments | 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%