Investments (Schedule of Realized Gain (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 83.8	$ 47.0	$ 225.1	$ 176.9
Gross Gains	0.8	2.4	2.0	6.4
Gross Losses	1.5	0.3	5.3	1.7
Fixed Maturities Held-For-Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	21.4	45.6	172.5	145.8
Gross Gains	0.2	0.6	1.3	1.8
Gross Losses	0.1	0.2	1.1	0.2
Fixed Maturities Including Held For Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	105.2	92.6	397.6	322.7
Gross Gains	1.0	3.0	3.3	8.2
Gross Losses	$ 1.6	$ 0.5	$ 6.4	$ 1.9