Investments - Net Realized Gains on Sales of Investments (Details) - Fixed Maturities: - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Investment Income [Line Items]
Realized Gains	$ 1.2	$ 3.5	$ 5.3	$ 4.9
Realized Losses	$ (1.9)	$ (0.2)	$ (4.0)	$ (0.4)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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