Investment Securities (Significant Unobservable Inputs) (Details) - NR $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate		9.00%
Kudu Investments | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 219.3
Kudu Investments | Minimum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	15.00%
Kudu Investments | Maximum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	20.00%
MediaAlpha | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 150.0
MediaAlpha | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	17.00%
MediaAlpha | Long-term revenue growth
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	4.00%
DavidShield & PassportCard | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 90.0	$ 75.0
DavidShield & PassportCard | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%	18.00%
DavidShield & PassportCard | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input	1	1
Private Debt Instrument | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 22.7	$ 10.0
Private Debt Instrument | Minimum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	6.00%
Private Debt Instrument | Maximum | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	9.00%
Elementum Holdings, L.P. | Price of Recent Transaction
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 55.1
Other | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 29.3	$ 30.0
Other | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%	23.00%
Other | Minimum | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input	2	2
Other | Maximum | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input	4	4
Compare.com | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value		$ 16.9
Compare.com | Estimated Net Realizable Value
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 2.5
Compare.com | Discount rate
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate		22.00%
Compare.com | Revenue exit multiple
Quantitative information for Level 3 Fair Value Measurements Assets
Equity Securities, FV-NI, Measurement Input		3
Galvanic Applied Sciences | Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 2.7	$ 3.1
Equity Securities, FV-NI, Measurement Input	6	6