Schedule III and VI Supplementary Insurance Information Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	$ 336.3	$ 330.6	$ 324.4
Future policy benefits, claims and claim expenses	7,530.5	7,542.6	7,477.4
Unearned premiums	300.9	266.1	255.7
Other policy claims and benefits payable	916.0	809.3	945.9
Premium revenue/ premium earned	1,057.1	1,027.7	888.8
Net investment income	444.8	400.9	422.5
Benefits, claims and settlement expenses	974.8	920.4	750.7
Amortization of deferred policy acquisition costs	101.2	88.2	90.6
Other operating expenses	362.6	356.5	277.9
Net premiums written (excluding life)	909.3	860.2	731.7
Property & Casualty
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	29.3	24.6	24.4
Future policy benefits, claims and claim expenses	416.9	388.7	367.4
Unearned premiums	297.9	259.1	249.8
Other policy claims and benefits payable	0.0	0.0	0.0
Premium revenue/ premium earned	645.5	608.2	617.4
Net investment income	37.9	31.4	61.1
Benefits, claims and settlement expenses	557.0	534.3	447.9
Amortization of deferred policy acquisition costs	71.3	64.3	67.7
Other operating expenses	103.3	102.6	97.2
Net premiums written (excluding life)	684.4	617.5	607.8
Life & Retirement
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	297.7	299.5	295.1
Future policy benefits, claims and claim expenses	6,418.2	6,413.7	6,714.6
Unearned premiums	0.0	4.0	2.8
Other policy claims and benefits payable	816.0	719.8	873.4
Premium revenue/ premium earned	151.8	144.0	143.4
Net investment income	369.9	338.3	338.6
Benefits, claims and settlement expenses	325.1	293.6	269.7
Amortization of deferred policy acquisition costs	28.0	23.0	22.1
Other operating expenses	99.0	108.2	102.0
Net premiums written (excluding life)	29.2	29.5	30.6
Supplemental & Group Benefits
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	9.3	6.5	4.9
Future policy benefits, claims and claim expenses	695.4	740.2	395.4
Unearned premiums	3.0	3.0	3.1
Other policy claims and benefits payable	100.0	89.5	72.5
Premium revenue/ premium earned	259.8	275.5	128.0
Net investment income	38.9	33.3	25.2
Benefits, claims and settlement expenses	92.7	92.5	33.1
Amortization of deferred policy acquisition costs	1.9	0.9	0.8
Other operating expenses	123.2	118.1	54.3
Net premiums written (excluding life)	195.7	213.2	93.3
Other, including consolidating eliminations | Eliminations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net investment income	(1.9)	(2.1)	(2.4)
Other operating expenses	$ 37.1	$ 27.6	$ 24.4