Investment Securities - Investment Holding Common Equities/ Other Investments/ MediaAlpha (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 152.5
Gross Unrealized Gains	8.8
Gross Unrealized Losses	(0.5)
Net Foreign Currency Gains (Losses)	(1.2)
Common equity securities, at fair value	159.6
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	0.0	$ 0.0
Gross Unrealized Gains	316.4	802.2
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Common equity securities, at fair value	316.4	802.2
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,154.7	767.4
Gross Unrealized Gains	197.6	95.8
Gross Unrealized Losses	(62.9)	(78.1)
Net Foreign Currency Gains (Losses)	(3.9)	1.7
Common equity securities, at fair value	$ 1,285.5	$ 786.8