Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,063.6	$ 1,052.9	$ 658.1	$ 609.8	$ 605.9	$ 598.2	$ 582.5	$ 563.4	$ 3,384.4	$ 2,350.0	$ 2,220.0
Net Investment Income	91.3	92.0	78.4	79.2	82.6	85.9	77.1	81.6	340.9	327.2	298.3
Other Income	2.0	37.8	1.2	1.2	1.1	1.0	1.0	0.9	42.2	4.0	3.2
Total Revenues	1,094.7	1,195.5	741.9	693.0	697.3	690.3	684.4	651.4	3,725.1	2,723.4	2,521.9
Equity Securities, FV-NI	(64.3)				0.0				(64.3)	0.0	0.0
Net Realized Gains on Sales of Investments									26.4	56.5	33.1
Net Impairment Losses Recognized in Earnings	(2.2)	$ (1.8)	$ 0.0	$ (0.5)	(3.8)	$ (2.9)	$ (2.6)	$ (5.0)	(4.5)	(14.3)	(32.7)
Other Noncash Income (Expense)									40.6	6.2	6.3
Operating Segments [Member]
Segment Reporting Information [Line Items]
Earned Premiums									3,384.4	2,350.0	2,220.0
Total Revenues									3,726.9	2,675.0	2,515.2
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									730.7	730.3	741.5
Net Investment Income									61.8	58.9	48.6
Total Revenues									792.5	789.2	790.1
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									2,027.4	1,005.7	873.3
Net Investment Income									63.4		29.1
Other Income									2.4	1.1	0.5
Total Revenues									2,093.2	1,046.0	902.9
Operating Segments [Member] | Life and Health Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums									626.3	614.0	605.2
Net Investment Income									210.9	223.2	214.2
Other Income									4.0	2.6	2.8
Total Revenues									841.2	839.8	822.2
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Equity Securities, FV-NI	$ (64.3)				$ 0.0				(64.3)	0.0	0.0
Net Realized Gains on Sales of Investments									26.4	56.5	33.1
Net Impairment Losses Recognized in Earnings									$ (4.5)	(14.3)	$ (32.7)
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Net Investment Income										$ 39.2