Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Earned Premiums	$ 561.4	$ 558.9	$ 553.7	$ 546.0	$ 541.5	$ 536.7	$ 500.1	$ 431.3	$ 2,220.0	$ 2,009.6	$ 1,862.2
Net Investment Income	79.9	77.7	73.7	67.0	79.4	75.9	76.7	70.6	298.3	302.6	309.1
Other Income	1.0	0.8	0.6	0.8	1.4	0.8	0.6	0.9	3.2	3.7	1.4
Total Revenues	642.7	640.7	627.2	611.3	617.0	615.4	609.2	499.2	2,521.9	2,340.8	2,196.6
Net Realized Gains on Sales of Investments	9.1	11.6	5.6	6.8	9.4	5.3	34.0	3.4	33.1	52.1	39.1
Net Impairment Losses Recognized in Earnings	$ (8.7)	$ (8.3)	$ (6.4)	$ (9.3)	$ (14.7)	$ (3.3)	$ (2.2)	$ (7.0)	(32.7)	(27.2)	(15.2)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues									2,508.9	2,300.1	2,155.0
Operating Segments [Member] | Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									1,614.8	1,415.2	1,249.5
Net Investment Income									72.4	73.3	72.7
Other Income									0.5	0.6	0.5
Total Revenues									1,687.7	1,489.1	1,322.7
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									605.2	594.4	612.7
Net Investment Income									213.2	214.2	218.7
Other Income									2.8	2.4	0.9
Total Revenues									821.2	811.0	832.3
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other Income									12.6	15.8	17.7
Net Realized Gains on Sales of Investments									33.1	52.1	39.1
Net Impairment Losses Recognized in Earnings									$ (32.7)	$ (27.2)	$ (15.2)