Investments Offsetting for Derivatives and Securities Lending Balances (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Offsetting Derivative Assets
Gross Derivative Asset	$ 19.5	$ 32.7
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	19.5	32.7
Securities Received as Collateral on Derivative Assets	(4.2)	(7.3)
Cash collateral from counterparties	(15.3)	(25.4)
Derivative Assets Net of Collateral	0.0	0.0
Offsetting Securities Borrowed
Gross Securities Borrowed	159.2	178.5
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	159.2	178.5
Securities Received as Collateral on Securities Borrowed	(128.7)	(148.6)
Cash Received as Collateral on Securities Borrowed	(30.5)	(29.9)
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	178.7	211.2
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	178.7	211.2
Securities Received as Collateral on Financial Assets	(132.9)	(155.9)
Cash Received as Collateral on Financial Assets	(45.8)	(55.3)
Financial Assets Net of Collateral	0.0	0.0
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	68.1	99.5
Cash Collateral to counterparties	0.0	0.0
Offsetting Securities Loaned
Gross Securities Loaned	30.5	29.9
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	30.5	29.9
Securities Given as Collateral on Securities Loaned	(30.5)	(29.9)
Cash Given as Collateral on Securities Loaned	0.0	0.0
Securities Loaned Net of Collateral	0.0	0.0
Offsetting Financial Liabilities
Gross Financial Liability	82.7	82.7
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	82.7	82.7
Securities Given as Collateral on Financial Liabilities	(73.4)	(67.5)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	9.3	15.2
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	52.2	52.8
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	52.2	52.8
Securities Given as Collateral on Derivative Liabilities	(42.9)	(37.6)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 9.3	$ 15.2