Schedule III and VI Supplementary Insurance Information Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	$ 358.2	$ 347.2	$ 336.3
Future policy benefits, claims and claim expenses	7,240.8	7,292.3	7,530.5
Unearned premiums	372.1	344.2	300.9
Other policy claims and benefits payable	1,046.2	995.7	916.0
Premium revenue/ premium earned	1,228.4	1,146.0	1,057.1
Net investment income	464.3	445.7	444.8
Benefits, claims and settlement expenses	928.4	960.9	974.8
Amortization of deferred policy acquisition costs	124.5	111.1	101.2
Other operating expenses	447.3	394.6	362.6
Net premiums written (excluding life)	1,126.9	1,063.6	909.3
Property & Casualty
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	40.0	34.4	29.3
Future policy benefits, claims and claim expenses	404.4	420.6	416.9
Unearned premiums	368.4	340.7	297.9
Other policy claims and benefits payable	0.0	0.0	0.0
Premium revenue/ premium earned	802.6	736.5	645.5
Net investment income	57.1	46.0	37.9
Benefits, claims and settlement expenses	496.0	521.0	557.0
Amortization of deferred policy acquisition costs	98.2	84.6	71.3
Other operating expenses	125.8	115.8	103.3
Net premiums written (excluding life)	830.2	779.3	684.4
Life & Retirement
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	303.7	301.4	297.7
Future policy benefits, claims and claim expenses	6,209.0	6,222.7	6,418.2
Unearned premiums	0.0	0.0	0.0
Other policy claims and benefits payable	911.2	880.8	816.0
Premium revenue/ premium earned	158.6	154.6	151.8
Net investment income	366.5	363.6	369.9
Benefits, claims and settlement expenses	334.0	336.4	325.1
Amortization of deferred policy acquisition costs	24.3	24.6	28.0
Other operating expenses	119.5	110.0	99.0
Net premiums written (excluding life)	29.3	29.7	29.2
Supplemental & Group Benefits
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred policy acquisition costs	14.5	11.4	9.3
Future policy benefits, claims and claim expenses	627.4	649.0	695.4
Unearned premiums	3.7	3.5	3.0
Other policy claims and benefits payable	135.0	114.9	100.0
Premium revenue/ premium earned	267.2	254.9	259.8
Net investment income	36.8	38.1	38.9
Benefits, claims and settlement expenses	98.4	83.5	92.7
Amortization of deferred policy acquisition costs	2.0	1.9	1.9
Other operating expenses	141.1	124.9	123.2
Net premiums written (excluding life)	267.4	254.6	195.7
Other, including consolidating eliminations | Eliminations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net investment income	3.9	(2.0)	(1.9)
Benefits, claims and settlement expenses	0.0	20.0
Other operating expenses	$ 60.9	$ 43.9	$ 37.1