Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,337.7		$ 1,085.3	$ 2,538.5	$ 2,251.7
Net Investment Income	113.9		67.8	217.0	153.4
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	(7.7)			(23.1)	0.0
Other Income (Loss)	7.0		1.5	8.5	91.8
Total Revenues	1,507.7		1,230.9	2,859.7	2,459.9
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	40.8		71.6	93.0	(46.2)
Impairment Losses				(7.2)	(19.0)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,445.7		1,151.5	2,734.1	2,408.4
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	1,010.3		759.0	1,887.9	1,581.5
Net Investment Income	42.7		16.9	77.7	45.7
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	(3.7)			(11.0)
Other Income (Loss)	1.0		0.1	1.9	1.0
Total Revenues	1,050.3		776.0	1,956.5	1,628.2
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	163.2		163.6	325.4	344.5
Net Investment Income	19.5		4.3	35.4	14.0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	(2.0)			(6.1)
Other Income (Loss)	0.0	$ 0.1	0.1	0.0
Total Revenues	180.7		168.0	354.7	358.6
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	164.2		162.7	325.2	325.7
Net Investment Income	52.4		44.3	103.5	95.3
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	(2.0)			(6.0)
Other Income (Loss)	0.1		0.5	0.2	0.6
Total Revenues	214.7		207.5	422.9	421.6
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	40.8		71.6	93.0	(46.2)
Net Realized Gains on Sales of Investments	19.2		11.7	33.0	28.2
Impairment Losses	(3.2)		(7.0)	(7.2)	(19.0)
Other	$ 5.2		$ 3.1	$ 6.8	$ 88.5