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Investment Securities - Significant Unobservable Inputs (Details) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) $ / shares	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / shares
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in kudu participation contracts	$ 11.0	$ 21.0
Kudu Investments | Minimum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%		18.00%
Kudu Investments | Minimum | Terminal Multiple | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal Cash Flow Exit Multiple	7		7
Kudu Investments | Maximum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%		23.00%
Kudu Investments | Maximum | Terminal Multiple | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal Cash Flow Exit Multiple	12		12
PassportCard/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in kudu participation contracts			$ 15.0
Private debt instruments | Minimum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	4.00%		4.00%
Private debt instruments | Maximum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	8.00%		8.00%
All other | Minimum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	20.00%		20.00%
All other | Maximum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	24.00%		24.00%
New Market Solutions, LLC | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares	$ 9.9
Zillion Insurance Services
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in kudu participation contracts			$ 2.5
Kudu
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in kudu participation contracts	$ 10.9		118.2
Level 3 | Kudu Investments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	427.4		400.6
Level 3 | PassportCard/DavidShield | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 95.0		$ 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 instruments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 17.0		$ 17.1
Level 3 | All other | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 19.8		$ 18.8
Level 3 | All other | 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 | New Market Solutions, LLC | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 9.9		$ 9.9
Level 3 | New Market Solutions, LLC | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares			$ 9.9
Level 3 | Noblr, Inc. | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 8.7		$ 8.7
Level 3 | Noblr, Inc. | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares	$ 8.7
Level 3 | Noblr, Inc. | Recent Transaction | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares			$ 8.7
Level 3 | Zillion Insurance Services | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 5.0
Level 3 | Zillion Insurance Services | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 5.0
Level 3 | Zillion Insurance Services | Recent Transaction | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 5.0
Level 3 | Zillion Insurance Services | Zillion Insurance Services | Standard Poors NR Rating | Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 5.0

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