Investments - Net Realized Gains on Sales of Investments (Details) - Fixed Maturities: - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Realized Gains	$ 5.9	$ 2.4	$ 11.2	$ 7.3
Realized Losses	$ (2.5)	$ 0.0	$ (6.5)	$ (0.4)

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