Investments Offsetting for Derivatives and Securities Lending Balances (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Offsetting Derivative Assets
Gross Derivative Assets	$ 10.9	$ 10.8
Derivative Assets, Gross Liability Offset	0	0
Derivative Assets, Amount Not Offset Against Collateral	10.9	10.8
Obligation to Return Securities on Derivative Assets	(7.9)	(9.5)
Obligation to Return Cash on Derivative Assets	(2.8)	(1.1)
Derivative Assets, Amount Offset Against Collateral	0.2	0.2
Offsetting Securities Borrowed
Gross Securities Borrowed	187.4	201.6
Securities Borrowed, Liability Offset	0	0
Securities Borrowed	187.4	201.6
Obligation to Return Securities on Securities Borrowed	(125.8)	(125.1)
Obligation to Return Cash on Securities Borrowed	(61.6)	(76.5)
Securities Borrowed, Amount Offset Against Collateral	0	0
Offsetting Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed
Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed, Gross	198.3	212.4
Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed, Liability	0	0
Derivative Asset, Securities Purchased under Agreements to Resell, Securities Borrowed	198.3	212.4
Derivative Assets and Securities Lending Assets, Obligation to Return Securities	(133.7)	(134.6)
Derivative Assets and Securities Lending Assets, Obligation to Return Cash	(64.4)	(77.6)
Derivative Assets and Securities Lending Assets, Amount Offset Against Collateral	0.2	0.2
Offsetting Derivative Liabilities
Gross Derivative Liabilities	117.7	135.6
Derivative Liabilities, Gross Asset Offset	0	0
Derivative Liabilities, Amount Not Offset Against Collateral	117.7	135.6
Right to Reclaim Securities on Derivative Liabilities	(84.1)	(98.6)
Right to Reclaim Cash on Derivative Liabilities	0	0
Derivative Liability, Amount Offset Against Collateral	33.6	37.0
Offsetting Securities Loaned
Gross Securities Loaned	61.6	76.5
Securities Loaned, Asset	0	0
Securities Loaned	61.6	76.5
Right to Reclaim Securities on Securities Loaned	(61.6)	(76.5)
Right to Reclaim Cash on Securities Loaned	0	0
Securities Loaned, Amount Offset Against Collateral	0	0
Derivative Liabilities and Securities Lending Liabilities
Derivative Liability and Securities Lending Liabilities, Gross	179.3	212.1
Derivative Liability and Securities Lending Liabilities, Asset Offset	0	0
Derivative Liability and Securities Lending Liabilities, Securities Loaned	179.3	212.1
Derivative Liability and Securities Lending Liabilities, Right to Reclaim Securities	(145.7)	(175.1)
Derivative Liability and Securities Lending Liabilities, Right to Reclaim Cash	0	0
Derivative Liability and Securities Lending Liabilities, Amount Offset Against Collateral	$ 33.6	$ 37.0