Investment Securities - Significant Unobservable Inputs (Details) $ / shares in Units, $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 2,533.4	$ 2,569.4	$ 2,746.0	$ 2,328.7
Investments in kudu participation contracts	$ 57.5	$ 74.4
Marketing Technology (MediaAlpha) | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 180.0
Marketing Technology (MediaAlpha) | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate			15.00%
Marketing Technology (MediaAlpha) | 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%
Kudu Investments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 266.5
Kudu Investments | Minimum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%		15.00%
Kudu Investments | Minimum | Terminal Multiple | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal Cash Flow Exit Multiple	6		6
Kudu Investments | Maximum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%		22.00%
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 | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 90.0
PassportCard/DavidShield | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate			22.00%
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%
Elementum Holdings, L.P. | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 55.1
Elementum Holdings, L.P. | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate			20.00%
Terminal Revenue Growth Rate			4.00%
Private debt instruments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 23.7
Private debt instruments | Minimum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	5.00%		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	10.00%		9.00%
All other | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 23.7
All other | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Terminal Revenue Growth Rate			4.00%
All other | Minimum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	20.00%		25.00%
All other | Maximum | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	24.00%		32.00%
Noblr, Inc. | Standard Poors NR Rating | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 5.0
Noblr, Inc. | Recent transaction | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares			$ 2.17
Zillion Insurance Services | Standard Poors NR Rating | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 2.5
Zillion Insurance Services | Recent transaction | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			2.5
Compare.com | Standard Poors NR Rating | Estimated Net Realizable Value
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			2.5
Compare.com | Estimated Net Realizable Value | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value			$ 2.5
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	57.4
Passport/DavidShield
Quantitative information for Level 3 Fair Value Measurements Assets
Investments in kudu participation contracts	15.0
Level 3 | Marketing Technology (MediaAlpha) | Standard Poors NR Rating | Valuation Technique, Probability Weighted Expected Return Method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 475.0
Level 3 | Marketing Technology (MediaAlpha) | Weight Ascribed to DCF [Member] | Standard Poors NR Rating | Valuation Technique, Probability Weighted Expected Return Method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	0.35
Level 3 | Marketing Technology (MediaAlpha) | Weight Ascribed to IPO [Member] | Standard Poors NR Rating | Valuation Technique, Probability Weighted Expected Return Method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	0.65
Level 3 | Marketing Technology (MediaAlpha) | Discount rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	0.13
Level 3 | Marketing Technology (MediaAlpha) | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	0.04
Level 3 | Marketing Technology (MediaAlpha) | Measurement Input, Revenue Multiple [Member] | Standard Poors NR Rating | Valuation Technique, Probability Weighted Expected Return Method [Member]
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	2.00
Level 3 | Kudu Investments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 325.5
Level 3 | PassportCard/DavidShield | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 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%
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%
Level 3 | Elementum Holdings, L.P. | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 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	19.00%
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%
Level 3 | Private debt instruments | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 17.9
Level 3 | All other | Standard Poors NR Rating | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 18.1
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%
Level 3 | Noblr, Inc. | Standard Poors NR Rating | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 8.7
Level 3 | Noblr, Inc. | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Share price (in USD per share) | $ / shares	$ 2.17
Level 3 | Zillion Insurance Services | Standard Poors NR Rating | Recent transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value investments	$ 5.0
Level 3 | Zillion Insurance Services | Measurement Input, Long-term Revenue Growth Rate | Standard Poors NR Rating
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 5.0