Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,214.0	$ 1,206.5	$ 1,085.3	$ 1,166.4	$ 1,145.8	$ 1,135.2	$ 1,116.6	$ 1,074.8	$ 4,672.2	$ 4,472.4	$ 3,384.4
Net Investment Income	102.7	92.1	67.8	85.6	93.9	91.7	96.0	82.7	348.2	364.3	340.9
Other Income	1.9	0.9	1.5	90.3	3.7	7.2	22.7	1.9	94.6	35.5	42.2
Total Revenues	1,392.1	1,353.7	1,230.9	1,229.0	1,283.7	1,243.8	1,275.4	1,236.3	5,205.7	5,039.2	3,725.1
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	73.1	45.2	71.6	(117.8)	39.2	9.8	25.5	64.4	72.1	138.9	(64.3)
Impairment Losses	$ 0.5	$ (1.0)	$ (7.0)	$ (12.0)	$ (1.7)	$ (1.8)	$ (6.7)	$ (3.6)	(19.5)	(13.8)	(4.5)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues									5,025.3	4,845.9	3,726.9
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums									3,335.3	3,078.4	2,027.4
Net Investment Income									114.1	107.5	63.4
Other Income									1.8	7.0	2.4
Total Revenues									3,451.2	3,192.9	2,093.2
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums									688.2	750.3	730.7
Net Investment Income									37.7	44.1	61.8
Other Income									0.1
Total Revenues									726.0	794.4	792.5
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums									648.7	643.7	626.3
Net Investment Income									198.8	206.4	210.9
Other Income									0.6	8.5	4.0
Total Revenues									848.1	858.6	841.2
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities									72.1	138.9	(64.3)
Net Realized Gains on the Sales of Investments									38.1	41.9	26.4
Impairment Losses									(19.5)	(13.8)	(4.5)
Other									$ 89.7	$ 26.3	$ 40.6