Investment Securities (Significant Unobservable Inputs) (Details) - NR $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
MediaAlpha
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 114.7
DavidShield & PassportCard | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	75.0	$ 75.0
Compare.com | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	16.9	16.9
durchblicker | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	15.2	15.5
Captricity | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	14.5	14.5
Galvanic | Multiple of EBITDA
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	2.7	3.1
Private Debt Instrument | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	10.0	$ 10.0
Private Debt Instrument | Discounted cash flow
Quantitative information for Level 3 Fair Value Measurements Assets
Assets, fair value	$ 10.9
Unit price | MediaAlpha
Quantitative information for Level 3 Fair Value Measurements Assets
Share Price | $ / shares	$ 208.98
Discount rate | DavidShield & PassportCard
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	18.00%	18.00%
Discount rate | Compare.com
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	22.00%	22.00%
Discount rate | durchblicker
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%	23.00%
Discount rate | Captricity
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	23.00%	23.00%
Discount rate | Private Debt Instrument
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	9.23%	9.23%
Discount rate | Private Debt Instrument
Quantitative information for Level 3 Fair Value Measurements Assets
Fair value, discount rate	7.00%
Revenue exit multiple | DavidShield & PassportCard
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	1.00	1.00
Revenue exit multiple | Compare.com
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	2.75	2.75
Revenue exit multiple | durchblicker
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	2.25	2.25
Revenue exit multiple | Captricity
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	3.75	3.75
EBITDA multiple | Galvanic
Quantitative information for Level 3 Fair Value Measurements Assets
Measurement input	6.00	6.00