Derivative Instruments (Location and Amounts of Unrealized And Realized Gains (Losses) On Derivatives Recorded In The Condensed Consolidated Income Statements) (Details) - Not designated as hedging instruments - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	$ (17.5)	$ (22.7)	$ (38.1)
Foreign exchange gain (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	(2.2)	(7.3)	0.9
Foreign exchange gain (loss) | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	(2.2)	(7.3)	0.9
Net realized investment gains (losses)
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	(19.6)	(15.4)	(39.0)
Net realized investment gains (losses) | Put options
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	0.0	0.0	0.5
Net realized investment gains (losses) | Foreign exchange contracts
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	(11.9)	0.2	3.5
Net realized investment gains (losses) | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	(6.8)	(13.4)	(45.4)
Net realized investment gains (losses) | Interest rate futures
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	(0.9)	(2.2)	2.4
Other income (loss)
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	4.3	0.0	0.0
Other income (loss) | Insurance contract
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	7.4	0.0	0.0
Other income (loss) | Reinsurance contract
Derivative Instruments, Gain (Loss) [Line Items]
Total realized and unrealized losses (gains) on derivatives	$ (3.1)	$ 0.0	$ 0.0