Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Earned Premiums	$ 605.9	$ 598.2	$ 582.5	$ 563.4	$ 561.4	$ 558.9	$ 553.7	$ 546.0	$ 2,350.0	$ 2,220.0	$ 2,009.6
Net Investment Income	82.6	85.9	77.1	81.6	79.9	77.7	73.7	67.0	327.2	298.3	302.6
Other Income	1.1	1.0	1.0	0.9	1.0	0.8	0.6	0.8	4.0	3.2	3.7
Total Revenues	697.3	690.3	684.4	651.4	642.7	640.7	627.2	611.3	2,723.4	2,521.9	2,340.8
Net Realized Gains on Sales of Investments	11.5	8.1	26.4	10.5	9.1	11.6	5.6	6.8	56.5	33.1	52.1
Net Impairment Losses Recognized in Earnings	$ (3.8)	$ (2.9)	$ (2.6)	$ (5.0)	$ (8.7)	$ (8.3)	$ (6.4)	$ (9.3)	(14.3)	(32.7)	(27.2)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Revenues									2,669.5	2,508.9	2,300.1
Operating Segments [Member] | Property and Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									1,736.0	1,614.8	1,415.2
Net Investment Income									94.3	72.4	73.3
Other Income									1.1	0.5	0.6
Total Revenues									1,831.4	1,687.7	1,489.1
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									614.0	605.2	594.4
Net Investment Income									221.5	213.2	214.2
Other Income									2.6	2.8	2.4
Total Revenues									838.1	821.2	811.0
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Other Income									11.7	12.6	15.8
Net Realized Gains on Sales of Investments									56.5	33.1	52.1
Net Impairment Losses Recognized in Earnings									$ (14.3)	$ (32.7)	$ (27.2)