Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Offsetting Derivative Assets
Gross Derivative Assets	$ 21.5	$ 19.5
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	21.5	19.5
Securities Received as Collateral on Derivative Assets	(5.4)	(4.2)
Cash Collateral from Counterparties	(16.1)	(15.3)
Derivative Assets Net of Collateral	0.0	0.0
Offsetting Securities Borrowed
Gross Securities Borrowed	104.2	159.2
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	104.2	159.2
Securities Received as Collateral on Securities Borrowed	(88.2)	(128.7)
Cash Received as Collateral on Securities Borrowed	(16.0)	(30.5)
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting of Derivative Assets and Securities Borrowed
Gross Financial Assets	125.7	178.7
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	125.7	178.7
Securities Received as Collateral on Financial Assets	(93.6)	(132.9)
Cash Received as Collateral on Financial Assets	(32.1)	(45.8)
Financial Assets Net of Collateral	0.0	0.0
Offsetting Securities Loaned
Gross Securities Loaned	16.0	30.5
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	16.0	30.5
Securities Given as Collateral on Securities Loaned	(16.0)	(30.5)
Cash Given as Collateral on Securities Loaned	0.0	0.0
Securities Loaned Net of Collateral	0.0	0.0
Derivative Liabilities and Securities Lending Liabilities
Gross Financial Liability	67.4	82.7
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	67.4	82.7
Securities Given as Collateral on Financial Liabilities	(55.0)	(73.4)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	12.4	9.3
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	51.4	52.2
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	51.4	52.2
Cash Collateral to Counterparties	0.0	0.0
Securities Given as Collateral on Derivative Liabilities	(39.0)	(42.9)
Derivative Liabilities Net of Collateral	$ 12.4	$ 9.3