Investments Offsetting for Derivatives and Securities Lending Balances (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Offsetting Derivative Assets
Gross Derivative Asset	$ 28.0	$ 10.8
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0	0
Net Derivative Assets Reported in Balance Sheet	28.0	10.8
Securities Received as Collateral on Derivative Assets	(7.2)	(9.5)
Cash collateral from counterparties	(15.4)	(1.1)
Derivative Assets Net of Collateral	5.4	0.2
Offsetting Securities Borrowed
Gross Securities Borrowed	176.5	201.6
Liabilities Offsetting Securities Borrowed in Balance Sheet	0	0
Net Securities Borrowed Reported in Balance Sheet	176.5	201.6
Securities Received as Collateral on Securities Borrowed	(118.1)	(125.1)
Cash Received as Collateral on Securities Borrowed	(58.4)	(76.5)
Securities Borrowed Net of Collateral	0	0
Offsetting Financial Assets
Gross Financial Assets	204.5	212.4
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0	0
Net Financial Assets Reported in Balance Sheet	204.5	212.4
Securities Received as Collateral on Financial Assets	(125.3)	(134.6)
Cash Received as Collateral on Financial Assets	(73.8)	(77.6)
Financial Assets Net of Collateral	5.4	0.2
Offsetting Derivative Liabilities
Gross Derivative Liability	92.9	135.6
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0	0
Net Derivative Liabilities Reported in Balance Sheet	92.9	135.6
Securities Given as Collateral on Derivative Liabilities	(67.0)	(98.6)
Cash Collateral to counterparties	0	0
Derivative Liabilities Net of Collateral	25.9	37.0
Offsetting Securities Loaned
Gross Securities Loaned	58.4	76.5
Assets Offsetting Securities Loaned in Balance Sheet	0	0
Net Securities Loaned Reported in Balance Sheet	58.4	76.5
Securities Given as Collateral on Securities Loaned	(58.4)	(76.5)
Cash Given as Collateral on Securities Loaned	0	0
Securities Loaned Net of Collateral	0	0
Offsetting Financial Liabilities
Gross Financial Liability	151.3	212.1
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0	0
Net Financial Liabilities Reported in Balance Sheet	151.3	212.1
Securities Given as Collateral on Financial Liabilities	(125.4)	(175.1)
Cash Given as Collateral on Financial Liabilities	0	0
Financial Liabilities Net of Collateral	$ 25.9	$ 37.0