Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Common equity securities
Cost or Amortized Cost	$ 537.0	$ 483.5
Gross Unrealized Gains	100.6	62.3
Gross Unrealized Losses	0.0	(1.2)
Net Foreign Currency Gains (Losses)	(6.8)	(6.2)
Carrying Value	630.8	538.4
Other long-term investments
Cost or Amortized Cost	1,639.5	1,655.7
Gross Unrealized Gains	501.5	446.3
Other Long-Term Investments, Unrealized Loss	(102.6)	(94.5)
Net Foreign Currency Gains (Losses)	(16.0)	(9.3)
Carrying Value	2,022.4	1,998.2
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	59.2
Gross Unrealized Gains	176.0	195.7
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	$ 235.2	$ 254.9