Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 26.1	$ 9.9	$ 82.1	$ (240.7)
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	2.0	(2.0)	19.4	13.3
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(2.5)	3.4	9.2	2.1
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(0.7)	(0.9)	(2.2)	0.8
Level 3 Inputs | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.0	0.0	0.0	0.1
Level 3 Inputs | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (0.7)	$ (0.9)	$ (2.2)	$ 0.7