Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
Common equity securities
Cost or Amortized Cost	$ 353.7	$ 537.0
Gross Unrealized Gains	85.1	120.4
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	(1.2)	(7.4)
Carrying Value	437.6	650.0
Other long-term investments
Cost or Amortized Cost	2,063.3	1,748.7
Gross Unrealized Gains	663.7	546.4
Gross Unrealized Losses	(126.6)	(116.7)
Net Foreign Currency Gains (Losses)	(11.2)	(28.2)
Carrying Value	2,589.2	2,150.2
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	59.2
Gross Unrealized Gains	144.0	142.4
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	$ 203.2	$ 201.6