Schedule of Financial Instrument and Derivative Offsetting (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Offsetting Derivative Assets
Gross Derivative Assets	$ 53.2	$ 28.0
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0	0
Net Derivative Assets Reported in Balance Sheet	53.2	28.0
Securities Received as Collateral on Derivative Assets	(21.2)	(7.2)
Cash Collateral from Counterparties	(28.7)	(15.4)
Derivative Assets Net of Collateral	3.3	5.4
Offsetting Securities Borrowed
Gross Securities Borrowed	181.1	176.5
Liabilities Offsetting Securities Borrowed in Balance Sheet	0	0
Net Securities Borrowed Reported in Balance Sheet	181.1	176.5
Securities Received as Collateral on Securities Borrowed	(155.8)	(118.1)
Cash Received as Collateral on Securities Borrowed	(25.3)	(58.4)
Securities Borrowed Net of Collateral	0	0
Offsetting of Derivative Assets and Securities Borrowed
Gross Financial Assets	234.3	204.5
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0	0
Net Financial Assets Reported in Balance Sheet	234.3	204.5
Securities Received as Collateral on Financial Assets	(177.0)	(125.3)
Cash Received as Collateral on Financial Assets	(54.0)	(73.8)
Financial Assets Net of Collateral	3.3	5.4
Offsetting Derivative Liabilities
Gross Derivative Liability	71.8	92.9
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0	0
Net Derivative Liabilities Reported in Balance Sheet	71.8	92.9
Securities Given as Collateral on Derivative Liabilities	(49.3)	(67.0)
Cash Collateral to Counterparties	0	0
Derivative Liabilities Net of Collateral	22.5	25.9
Offsetting Securities Loaned
Gross Securities Loaned	25.3	58.4
Assets Offsetting Securities Loaned in Balance Sheet	0	0
Net Securities Loaned Reported in Balance Sheet	25.3	58.4
Securities Given as Collateral on Securities Loaned	(25.3)	(58.4)
Cash Given as Collateral on Securities Loaned	0	0
Securities Loaned Net of Collateral	0	0
Derivative Liabilities and Securities Lending Liabilities
Gross Financial Liability	97.1	151.3
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0	0
Net Financial Liabilities Reported in Balance Sheet	97.1	151.3
Securities Given as Collateral on Financial Liabilities	(74.6)	(125.4)
Cash Given as Collateral on Financial Liabilities	0	0
Financial Liabilities Net of Collateral	$ 22.5	$ 25.9