Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	$ 38.3	$ 27.5
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	38.3	27.5
Securities Received as Collateral on Derivative Assets	(4.2)	(4.0)
Cash Collateral from Counterparties	(34.1)	(23.5)
Derivative Assets Net of Collateral	0.0	0.0
Offsetting Securities Borrowed
Gross Securities Borrowed	0.0	176.4
Liabilities Offsetting Securities Borrowed in Balance Sheet		0.0
Net Securities Borrowed Reported in Balance Sheet		176.4
Securities Received as Collateral on Securities Borrowed		(176.4)
Cash Received as Collateral on Securities Borrowed		0.0
Securities Borrowed Net of Collateral		0.0
Offsetting Financial Assets
Gross Financial Assets		203.9
Financial Liabilities Offsetting Financial Assets in Balance Sheet		0.0
Net Financial Assets Reported in Balance Sheet		203.9
Securities Received as Collateral on Financial Assets		(180.4)
Cash Received as Collateral on Financial Assets		(23.5)
Financial Assets Net of Collateral		0.0
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	87.7	57.4
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	34.0	34.6
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	34.0	34.6
Securities Given as Collateral on Derivative Liabilities	(33.4)	(31.3)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 0.6	$ 3.3