Schedule III and VI Supplementary Insurance Information Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	$ 347.2	$ 336.3	$ 330.6
Future policy benefits, claims and claim expenses	7,292.3	7,530.5	7,542.6
Unearned premiums	344.2	300.9	266.1
Other policy claims and benefits payable	995.7	916.0	809.3
Premium revenue/ premium earned	1,146.0	1,057.1	1,027.7
Net investment income	445.7	444.8	400.9
Benefits, claims and settlement expenses	960.9	974.8	920.4
Amortization of deferred policy acquisition costs	111.1	101.2	88.2
Other operating expenses	394.6	362.6	356.5
Net premiums written (excluding life)	1,063.6	909.3	860.2
Property & Casualty
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	34.4	29.3	24.6
Future policy benefits, claims and claim expenses	420.6	416.9	388.7
Unearned premiums	340.7	297.9	259.1
Other policy claims and benefits payable	0.0	0.0	0.0
Premium revenue/ premium earned	736.5	645.5	608.2
Net investment income	46.0	37.9	31.4
Benefits, claims and settlement expenses	521.0	557.0	534.3
Amortization of deferred policy acquisition costs	84.6	71.3	64.3
Other operating expenses	115.8	103.3	102.6
Net premiums written (excluding life)	779.3	684.4	617.5
Life & Retirement
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	301.4	297.7	299.5
Future policy benefits, claims and claim expenses	6,222.7	6,418.2	6,413.7
Unearned premiums	0.0	0.0	4.0
Other policy claims and benefits payable	880.8	816.0	719.8
Premium revenue/ premium earned	154.6	151.8	144.0
Net investment income	363.6	369.9	338.3
Benefits, claims and settlement expenses	336.4	325.1	293.6
Amortization of deferred policy acquisition costs	24.6	28.0	23.0
Other operating expenses	110.0	99.0	108.2
Net premiums written (excluding life)	29.7	29.2	29.5
Supplemental & Group Benefits
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	11.4	9.3	6.5
Future policy benefits, claims and claim expenses	649.0	695.4	740.2
Unearned premiums	3.5	3.0	3.0
Other policy claims and benefits payable	114.9	100.0	89.5
Premium revenue/ premium earned	254.9	259.8	275.5
Net investment income	38.1	38.9	33.3
Benefits, claims and settlement expenses	83.5	92.7	92.5
Amortization of deferred policy acquisition costs	1.9	1.9	0.9
Other operating expenses	124.9	123.2	118.1
Net premiums written (excluding life)	254.6	195.7	213.2
Other, including consolidating eliminations | Eliminations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net investment income	(2.0)	(1.9)	(2.1)
Benefits, claims and settlement expenses	20.0
Other operating expenses	$ 43.9	$ 37.1	$ 27.6