Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Common equity securities
Cost or Amortized Cost	$ 537.0	$ 483.5
Gross Unrealized Gains	120.4	62.3
Gross Unrealized Losses	0.0	(1.2)
Net Foreign Currency Gains (Losses)	(7.4)	(6.2)
Carrying Value	650.0	538.4
Other long-term investments
Cost or Amortized Cost	1,748.7	1,655.7
Gross Unrealized Gains	546.4	446.3
Gross Unrealized Losses	(116.7)	(94.5)
Net Foreign Currency Gains (Losses)	(28.2)	(9.3)
Carrying Value	2,150.2	1,998.2
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	59.2
Gross Unrealized Gains	142.4	195.7
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	$ 201.6	$ 254.9