Investment Securities - Common Equity Securities and Other Long Term Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Common equity securities
Cost or Amortized Cost	$ 599.5	$ 660.6
Gross Unrealized Gains	51.6	26.7
Gross Unrealized Losses	(4.8)	(8.4)
Net Foreign Currency Gains (Losses)	(9.3)	(10.5)
Carrying Value	637.0	668.4
Other long-term investments
Cost or Amortized Cost	1,639.9	1,340.8
Gross Unrealized Gains	390.9	271.1
Other Long-Term Investments, Unrealized Loss	(116.6)	(107.1)
Net Foreign Currency Gains (Losses)	(18.4)	(16.8)
Carrying Value	1,895.8	1,488.0
MediaAlpha
Investment in MediaAlpha
Cost or Amortized Cost	59.2	0.0
Gross Unrealized Gains	139.9	168.6
Gross Unrealized Losses	(10.3)	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	$ 188.8	$ 168.6