Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,116.6	$ 658.1	$ 2,191.4	$ 1,267.9
Net Investment Income	96.0	78.4	178.7	157.6
Other Income	22.7	1.2	24.6	2.4
Total Revenues	1,275.4	741.9	2,511.7	1,434.9
Income from Change in Fair Value of Equity and Convertible Securities	25.5	0.4	89.9	1.1
Net Impairment Losses Recognized in Earnings	(6.7)	0.0	(10.3)	(0.5)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,213.4	738.1	2,371.6	1,426.3
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	188.5	181.1	374.1	357.9
Net Investment Income	12.3	14.0	20.6	27.7
Total Revenues	200.8	195.1	394.7	385.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	766.0	320.0	1,495.3	598.4
Net Investment Income	28.9	10.2	50.4	20.1
Other Income	1.0	0.4	1.8	0.7
Total Revenues	795.9	330.6	1,547.5	619.2
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	162.1	157.0	322.0	311.6
Net Investment Income	53.0	54.5	104.7	108.2
Other Income	1.6	0.9	2.7	1.7
Total Revenues	216.7	212.4	429.4	421.5
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income from Change in Fair Value of Equity and Convertible Securities	25.5	0.4	89.9	1.1
Net Realized Gains on Sales of Investments	21.3	3.8	37.4	6.4
Net Impairment Losses Recognized in Earnings	(6.7)	0.0	(10.3)	(0.5)
Other	$ 21.9	$ (0.4)	$ 23.1	$ 1.6