Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	$ 1,166.9	$ 1,337.6	$ 2,347.8	$ 2,657.6
Net Investment Income	106.3	118.5	208.1	218.5
Change in Value of Alternative Energy Partnership Investments	0.8	(4.9)	1.5	(21.6)
Other Income	1.7	0.9	2.9	3.3
Total Revenues	1,262.8	1,417.7	2,557.6	2,787.8
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	2.4	(40.5)	4.1	(68.7)
Impairment Gains (Losses)	(0.9)	(4.9)	1.2	(13.8)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,273.2	1,440.7	2,554.1	2,842.5
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	932.3	1,043.7	1,876.5	2,065.3
Net Investment Income	44.5	34.0	83.0	68.9
Change in Value of Alternative Energy Partnership Investments	0.4	(2.5)	0.8	(10.9)
Other Income	0.7	1.0	1.6	2.7
Total Revenues	977.9	1,076.2	1,961.9	2,126.0
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	132.4	149.9	269.8	305.5
Net Investment Income	13.1	11.9	23.6	24.4
Change in Value of Alternative Energy Partnership Investments	0.2	(1.1)	0.3	(5.0)
Total Revenues	145.7	160.7	293.7	324.9
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Six and Three Months Ended: 2023 - $35.7 and $16.5; 2022 - $37.7 and $17.4)	102.2	144.0	201.5	286.8
Net Investment Income	47.1	61.9	96.9	111.3
Change in Value of Alternative Energy Partnership Investments	0.2	(1.3)	0.4	(5.7)
Other Income	0.1	(0.8)	(0.3)	(0.8)
Total Revenues	149.6	203.8	298.5	391.6
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	2.4	(40.5)	4.1	(68.7)
Gain (Loss) on Sale of Investments	(14.4)	11.0	(8.0)	12.5
Impairment Gains (Losses)			1.2	(13.8)
Net Impairment Gains (Losses) Recognized in Earnings	(0.9)	(4.9)
Other	$ 2.5	$ 11.4	$ 6.2	$ 15.3