Schedule of Financial Instrument and Derivative Offsetting (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Offsetting Derivative Assets
Gross Derivative Assets	$ 15.1	$ 10.8
Gross Derivatives Offset Against Derivative Assets in Balance Sheet	0	0
Net Derivative Asset Amount Presented in Balance Sheet	15.1	10.8
Obligation to Return Securities on Derivative Assets	(6.3)	(9.5)
Obligation to Return Cash on Derivative Assets	(8.1)	(1.1)
Net Amount of Derivative Assets	0.7	0.2
Offsetting Securities Borrowed
Gross Securities Lending Assets	174.1	201.6
Amount Offset Against Securities Lending Assets in Balance Sheet	0	0
Net Amount of Securities Lending Assets Presented in Balance Sheet	174.1	201.6
Obligation to Return Securities on Securities Lending Assets	(103.9)	(125.1)
Obligation to Return Cash on Securities Lending Assets	(70.2)	(76.5)
Net Amount of Securities Lending Assets	0	0
Offsetting of Derivative Assets and Securities Borrowed
Gross Derivative Assets and Securities Lending Assets	189.2	212.4
Gross Offset Against Financial Assets in Balance Sheet	0	0
Net Derivative Assets and Securities Lending Assets Presented in Balance Sheet	189.2	212.4
Derivative Assets and Securities Lending Assets, Obligation to Return Securities	(110.2)	(134.6)
Derivative Assets and Securities Lending Assets, Obligation to Return Cash	(78.3)	(77.6)
Net Amount of Derivative Assets and Securities Lending Assets	0.7	0.2
Offsetting Derivative Liabilities
Gross Derivative Liabilities	107.2	135.6
Gross Derivatives Offset Against Derivative Liabilities in Balance Sheet	0	0
Net Derivative Liability Amount Presented in Balance Sheet	107.2	135.6
Right to Reclaim Securities on Derivative Liabilities	(76.9)	(98.6)
Right to Reclaim Cash on Derivative Liabilities	0	0
Net Amount of Derivative Liabilities	30.3	37.0
Offsetting Securities Loaned
Gross Securities Lending Liability	70.2	76.5
Securities Lending Assets Offset Against Securities Lending Liabilities in Balance Sheet	0	0
Net Securities Lending Liability Presented in Balance Sheet	70.2	76.5
Right to Reclaim Securities on Securities Loaned	(70.2)	(76.5)
Right to Reclaim Cash on Securities Loaned	0	0
Net Amount of Securities Lending Liability	0	0
Derivative Liabilities and Securities Lending Liabilities
Gross Derivative Liability and Securities Lending Liability	177.4	212.1
Gross Offset Against Financial Liabilities in Balance Sheet	0	0
Net Derivative Liability and Securities Lending Liability Presented in Balance Sheet	177.4	212.1
Derivative Liability and Securities Lending Liabilities, Right to Reclaim Securities	(147.1)	(175.1)
Derivative Liability and Securities Lending Liabilities, Right to Reclaim Cash	0	0
Net Amount of Derivative Liability and Securities Lending Liability	$ 30.3	$ 37.0