Investment Securities (Investment Gains (Losses) For Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Realized gains	$ 49.4	$ 61.5	$ 283.7
Realized losses	62.3	37.4	13.7
Unrealized investment gains	(83.6)	97.5	(294.7)
Level 3 Inputs
Investment securities
Unrealized investment gains	22.6	(15.4)	(14.2)
Fixed maturity investments
Investment securities
Unrealized investment gains	(8.3)	13.8	(14.6)
Fixed maturity investments | Level 3 Inputs
Investment securities
Unrealized investment gains	0.0	0.0	0.1
Common equity securities
Investment securities
Unrealized investment gains	(105.5)	99.3	(257.4)
Other long-term investments
Investment securities
Unrealized investment gains	30.2	(15.6)	(22.7)
Other long-term investments | Level 3 Inputs
Investment securities
Unrealized investment gains	$ 22.6	$ (15.4)	$ (14.3)