Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Offsetting Derivative Assets
Gross Derivative Assets	$ 34.9	$ 30.9
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	34.9	30.9
Securities Received as Collateral on Derivative Assets	(6.1)	(6.9)
Cash Collateral from Counterparties	(28.8)	(24.0)
Derivative Assets Net of Collateral	0.0	0.0
Offsetting Securities Borrowed
Gross Securities Borrowed	153.6	164.1
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	153.6	164.1
Securities Received as Collateral on Securities Borrowed	(151.8)	(164.0)
Cash Received as Collateral on Securities Borrowed	(1.8)	(0.1)
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting of Derivative Assets and Securities Borrowed
Gross Financial Assets	188.5	195.0
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	188.5	195.0
Securities Received as Collateral on Financial Assets	(157.9)	(170.9)
Cash Received as Collateral on Financial Assets	(30.6)	(24.1)
Financial Assets Net of Collateral	0.0	0.0
Offsetting Securities Loaned
Gross Securities Loaned	1.8	0.1
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	1.8	0.1
Securities Given as Collateral on Securities Loaned	(1.8)	(0.1)
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	35.7	38.1
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	35.7	38.1
Securities Given as Collateral on Financial Liabilities	(34.2)	(33.3)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	1.5	4.8
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	33.9	38.0
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	33.9	38.0
Securities Given as Collateral on Derivative Liabilities	(32.4)	(33.2)
Cash Collateral to Counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 1.5	$ 4.8