Investments Schedule of Financial Instrument and Derivative Offsetting (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Offsetting Derivative Assets
Derivative Asset, Fair Value, Gross Asset	$ 27.5	$ 30.9
Derivative Liabilities Offsetting Derivative Assets in Balance Sheet	0.0	0.0
Net Derivative Assets Reported in Balance Sheet	27.5	30.9
Securities Received as Collateral on Derivative Assets	(4.0)	(6.9)
Cash Collateral from Counterparties	(23.5)	(24.0)
Derivative Assets Net of Collateral	0.0	0.0
Offsetting Securities Borrowed
Gross Securities Borrowed	176.4	164.1
Liabilities Offsetting Securities Borrowed in Balance Sheet	0.0	0.0
Net Securities Borrowed Reported in Balance Sheet	176.4	164.1
Securities Received as Collateral on Securities Borrowed	(176.4)	(164.0)
Cash Received as Collateral on Securities Borrowed	0.0	(0.1)
Securities Borrowed Net of Collateral	0.0	0.0
Offsetting Financial Assets
Gross Financial Assets	203.9	195.0
Financial Liabilities Offsetting Financial Assets in Balance Sheet	0.0	0.0
Net Financial Assets Reported in Balance Sheet	203.9	195.0
Securities Received as Collateral on Financial Assets	(180.4)	(170.9)
Cash Received as Collateral on Financial Assets	(23.5)	(24.1)
Financial Assets Net of Collateral	0.0	0.0
Offsetting Derivative Liabilities
Net Derivative Liabilities Reported in Balance Sheet	57.4	69.1
Offsetting Securities Loaned
Gross Securities Loaned		0.1
Assets Offsetting Securities Loaned in Balance Sheet		0.0
Net Securities Loaned Reported in Balance Sheet		0.1
Securities Given as Collateral on Securities Loaned		(0.1)
Cash Given as Collateral on Securities Loaned		0.0
Securities Loaned Net of Collateral		0.0
Offsetting Financial Liabilities
Gross Financial Liability		38.1
Financial Assets Offsetting Financial Liabilities in Balance Sheet		0.0
Net Financial Liabilities Reported in Balance Sheet		38.1
Securities Given as Collateral on Financial Liabilities		(33.3)
Cash Given as Collateral on Financial Liabilities		0.0
Financial Liabilities Net of Collateral		4.8
Over the Counter
Offsetting Derivative Liabilities
Gross Derivative Liability	34.6	38.0
Derivative Assets Offsetting Derivative Liabilities in Balance Sheet	0.0	0.0
Net Derivative Liabilities Reported in Balance Sheet	34.6	38.0
Securities Given as Collateral on Derivative Liabilities	(31.3)	(33.2)
Cash Collateral to counterparties	0.0	0.0
Derivative Liabilities Net of Collateral	$ 3.3	$ 4.8