Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,175.4	$ 4,250.2
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	537.9	440.8
Trading Securities, Unrealized Holding Gain	65.0	35.9
Trading Securities, Unrealized Holding Loss	(2.4)	(2.4)
Net foreign currency gains (losses)	0.5	0.0
Fixed maturity investments, at fair value	601.0	474.3
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	321.9	314.9
Trading Securities, Unrealized Holding Gain	39.7	40.3
Trading Securities, Unrealized Holding Loss	(23.3)	(28.0)
Net foreign currency gains (losses)	(6.6)	(3.9)
Fixed maturity investments, at fair value	$ 331.7	$ 323.3