Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	$ 835.9	$ 739.7
Trading Securities, Unrealized Holding Gain	111.7	129.4
Trading Securities, Unrealized Holding Loss	(2.0)	(3.0)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	945.6	866.1
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	288.8	246.6
Trading Securities, Unrealized Holding Gain	10.0	6.8
Trading Securities, Unrealized Holding Loss	(44.8)	(39.7)
Net foreign currency gains (losses)	(0.9)	(4.9)
Fixed maturity investments, at fair value	$ 253.1	$ 208.8