Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Earned Premiums	$ 541.5	$ 536.7	$ 500.1	$ 431.3	$ 451.5	$ 462.8	$ 470.3	$ 477.6	$ 2,009.6	$ 1,862.2	$ 2,025.8
Net Investment Income	79.4	75.9	76.7	70.6	93.1	72.3	72.6	71.1	302.6	309.1	314.7
Other Income	1.4	0.8	0.6	0.9	0.6	0.5	0.2	0.1	3.7	1.4	0.8
Total Revenues	617.0	615.4	609.2	499.2	559.7	539.8	542.5	554.6	2,340.8	2,196.6	2,426.5
Net Realized Gains on Sales of Investments	9.4	5.3	34.0	3.4	21.0	8.0	3.5	6.6	52.1	39.1	99.1
Net Impairment Losses Recognized in Earnings	$ (14.7)	$ (3.3)	$ (2.2)	$ (7.0)	$ (6.5)	$ (3.8)	$ (4.1)	$ (0.8)	(27.2)	(15.2)	(13.9)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues									2,300.1	2,155.0	2,327.3
Operating Segments [Member] | Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									1,415.2	1,249.5	1,392.9
Net Investment Income									73.3	72.7	90.9
Other Income									0.6	0.5	0.5
Total Revenues									1,489.1	1,322.7	1,484.3
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									594.4	612.7	632.9
Net Investment Income									214.2	218.7	209.9
Other Income									2.4	0.9	0.2
Total Revenues									811.0	832.3	843.0
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other Income									15.8	17.7	14.0
Net Realized Gains on Sales of Investments									52.1	39.1	99.1
Net Impairment Losses Recognized in Earnings									$ (27.2)	$ (15.2)	$ (13.9)