Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment securities
Sales and maturities of investment securities	$ 1,000.0	$ 2,200.0	$ 2,800.0
Common Stock
Investment securities
Cost or Amortized Cost	553.3	904.7
Gross Unrealized Gains	130.6	51.0
Gross Unrealized Losses	0.0	(30.1)
Net Foreign Currency Gains	0.0	0.0
Fixed maturity investments, at fair value	683.9	925.6
Other long-term investments
Investment securities
Cost or Amortized Cost	667.4	330.3
Gross Unrealized Gains	255.2	52.2
Gross Unrealized Losses	(64.1)	(54.9)
Net Foreign Currency Gains	(2.2)	(2.0)
Fixed maturity investments, at fair value	$ 856.3	$ 325.6