Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Common equity securities
Cost or Amortized Cost	$ 483.5	$ 660.6
Gross Unrealized Gains	62.3	26.7
Gross Unrealized Losses	(1.2)	(8.4)
Net Foreign Currency Gains (Losses)	(6.2)	(10.5)
Carrying Value	538.4	668.4
Other long-term investments
Cost or Amortized Cost	1,655.7	1,340.8
Gross Unrealized Gains	446.3	271.1
Gross Unrealized Losses	(94.5)	(107.1)
Net Foreign Currency Gains (Losses)	(9.3)	(16.8)
Carrying Value	1,998.2	1,488.0
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	0.0
Gross Unrealized Gains	195.7	168.6
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	$ 254.9	$ 168.6