Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	$ 30.5	$ 19.8
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	30.5	19.8
Securities Received as Collateral on Derivative Assets	(7.5)	(10.1)
Cash Collateral from Counterparties	(22.4)	(8.7)
Derivative Assets Net of Collateral	0.6	1.0
Offsetting Securities Borrowed
Gross Securities Borrowed	256.3	96.6
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	256.3	96.6
Securities Received as Collateral on Securities Borrowed	(151.2)	(79.0)
Cash Received as Collateral on Securities Borrowed	(105.1)	(17.6)
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	286.8	116.4
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	286.8	116.4
Securities Received as Collateral on Financial Assets	(158.7)	(89.1)
Cash Received as Collateral on Financial Assets	(127.5)	(26.3)
Financial Assets Net of Collateral	0.6	1.0
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	54.0	99.5
Offsetting Securities Loaned
Gross Securities Loaned	105.1	17.6
Assets Offsetting Securities Loaned in Balance Sheet	0.0	0.0
Net Securities Loaned Reported in Balance Sheet	105.1	17.6
Securities Given as Collateral on Securities Loaned	(105.1)	(17.6)
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	140.5	77.3
Financial Assets Offsetting Financial Liabilities in Balance Sheet	0.0	0.0
Net Financial Liabilities Reported in Balance Sheet	140.5	77.3
Securities Given as Collateral on Financial Liabilities	(140.0)	(76.6)
Cash Given as Collateral on Financial Liabilities	0.0	0.0
Financial Liabilities Net of Collateral	0.5	0.7
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	35.4	59.7
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	35.4	59.7
Securities Given as Collateral on Derivative Liabilities	(34.9)	(59.0)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 0.5	$ 0.7