Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Common equity securities
Cost or Amortized Cost	$ 537.0	$ 483.5
Gross Unrealized Gains	120.2	62.3
Gross Unrealized Losses	0.0	(1.2)
Net Foreign Currency Gains (Losses)	(1.8)	(6.2)
Carrying Value	655.4	538.4
Other long-term investments
Cost or Amortized Cost	1,638.6	1,655.7
Gross Unrealized Gains	547.9	446.3
Other Long-Term Investments, Unrealized Loss	(108.3)	(94.5)
Net Foreign Currency Gains (Losses)	(7.1)	(9.3)
Carrying Value	2,071.1	1,998.2
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	59.2
Gross Unrealized Gains	264.2	195.7
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	$ 323.4	$ 254.9