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Investment Securities - Significant Unobservable Inputs (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Contract $ / shares	Dec. 31, 2019 USD ($) $ / shares Contract	Dec. 31, 2018 USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Trading securities	$ 118.3	$ 118.4	$ 0.0
Kudu’s Participation Contracts (5) | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Cash flow exit multiple | Contract	7	6
Kudu’s Participation Contracts (5) | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Cash flow exit multiple | Contract	12	12
MediaAlpha | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)		4.00%
DavidShield & PassportCard
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Trading securities	$ 15.0
DavidShield & PassportCard | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)		4.00%
Elementum Holdings, L.P.
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Trading securities		$ 55.1
Elementum Holdings, L.P. | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)		4.00%
All other | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)		4.00%
Zillion Insurance Services, Inc. (7)
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Trading securities	2.5	$ 2.5
Kudu
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in Trading securities	$ 118.2	$ 198.0
Measurement Input, Discount Rate | Kudu’s Participation Contracts (5) | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	18.00%	15.00%
Measurement Input, Discount Rate | Kudu’s Participation Contracts (5) | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	23.00%	22.00%
Measurement Input, Discount Rate | MediaAlpha | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)		15.00%
Measurement Input, Discount Rate | DavidShield & PassportCard | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	23.00%	22.00%
Measurement Input, Discount Rate | Elementum Holdings, L.P. | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	17.00%	20.00%
Terminal revenue growth rate (as a percent)	4.00%
Measurement Input, Discount Rate | Private debt instruments | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	4.00%	4.00%
Measurement Input, Discount Rate | Private debt instruments | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	8.00%	9.00%
Measurement Input, Discount Rate | All other | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)	4.00%
Measurement Input, Discount Rate | All other | Standard Poors NR Rating | Minimum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	20.00%	25.00%
Measurement Input, Discount Rate | All other | Standard Poors NR Rating | Maximum
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate (as a percent)	24.00%	32.00%
Measurement Input, Long-term Revenue Growth Rate | DavidShield & PassportCard | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal revenue growth rate (as a percent)	4.00%
Measurement Input, Long-term Revenue Growth Rate | Noblr, Inc. | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares	$ 8.7
Measurement Input, Long-term Revenue Growth Rate | Zillion Insurance Services, Inc. (7) | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 5.0
Measurement Input, Long-term Revenue Growth Rate | New Market Solutions, LLC | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares	$ 9.9
Discounted Cash Flow Valuation Technique | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 23.7
Discounted Cash Flow Valuation Technique | Kudu’s Participation Contracts (5) | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 400.6	266.5
Discounted Cash Flow Valuation Technique | MediaAlpha | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		180.0
Discounted Cash Flow Valuation Technique | DavidShield & PassportCard | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	95.0	90.0
Discounted Cash Flow Valuation Technique | Elementum Holdings, L.P. | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	55.1	55.1
Discounted Cash Flow Valuation Technique | Private debt instruments | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	17.1
Discounted Cash Flow Valuation Technique | All other | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	18.8
Recent Transaction | Noblr, Inc. | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	8.7	$ 5.0
Share price (in USD per share) | $ / shares		$ 5.0
Recent Transaction | Zillion Insurance Services, Inc. (7) | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	5.0	$ 2.5
Investment, measurement input | $ / shares		2.5
Recent Transaction | New Market Solutions, LLC | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 9.9
Estimated Net Realizable Value | Compare.com | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 2.5
Investment, measurement input | $ / shares		2.5

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