Segment Information - Summarized Financial Information by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount	$ 1,029.0	$ 889.6	$ 930.7
Net investment income	400.9	422.5	357.6
Net income (loss)	(2.6)	142.8	133.3
Assets	13,446.8	14,383.9	13,471.8
Intersegment eliminations
Summarized financial information for these segments
Net investment income	(2.1)	(2.3)	(2.2)
Assets	(64.8)	(64.8)	(90.1)
Property & Casualty | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount	608.2	617.4	650.1
Net investment income	31.4	61.1	42.6
Net income (loss)	(44.4)	57.0	76.5
Assets	1,083.8	1,243.4	1,324.9
Life & Retirement | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount	145.3	144.2	147.0
Net investment income	338.3	338.6	299.3
Net income (loss)	48.8	68.4	30.7
Assets	10,858.3	12,068.6	11,243.2
Supplemental & Group Benefits | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned, Net Amount	275.5	128.0	133.6
Net investment income	33.3	25.2	18.1
Net income (loss)	58.5	46.0	42.9
Assets	1,396.1	854.9	811.5
Corporate & Other
Summarized financial information for these segments
Net investment income	0.0	(0.1)	(0.2)
Net income (loss)	(65.5)	(28.6)	(16.8)
Assets	$ 173.4	$ 281.8	$ 182.3