Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Unrealized investment gains	$ 97.5	$ (294.7)	$ 235.5
Foreign Currency Transaction Gain (Loss), Unrealized	11.7	(2.7)	(5.0)
Net unrealized investment gains (losses), pre-tax	109.2	(297.4)	230.5
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(4.0)	48.3	(41.3)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(4.0)	48.3	(41.3)
Unrealized investment gains after tax	93.5	(246.4)	194.2
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	11.7	(2.7)	(5.0)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	105.2	(249.1)	189.2
Fixed maturity investments
Investment securities
Unrealized investment gains	13.8	(14.6)	0.1
Foreign Currency Transaction Gain (Loss), Unrealized	12.7	2.1	0.0
Net unrealized investment gains (losses), pre-tax	26.5	(12.5)	0.1
Common equity securities
Investment securities
Unrealized investment gains	99.3	(257.4)	240.8
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	(3.3)	(3.9)
Net unrealized investment gains (losses), pre-tax	99.3	(260.7)	236.9
Other long-term investments
Investment securities
Unrealized investment gains	(15.6)	(22.7)	(5.4)
Foreign Currency Transaction Gain (Loss), Unrealized	(1.0)	(1.5)	(1.1)
Net unrealized investment gains (losses), pre-tax	$ (16.6)	$ (24.2)	$ (6.5)