Segment Information - Summarized Financial Information by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Summarized financial information for these segments
Premiums and contract charges earned	$ 889.6	$ 930.7	$ 898.0
Net investment income	422.5	357.6	365.1
Net income	142.8	133.3	184.4
Assets	14,383.9	13,471.8	12,478.7
Intersegment eliminations
Summarized financial information for these segments
Net investment income	(2.3)	(2.2)	(1.5)
Assets	(64.8)	(90.1)	(64.1)
Property & Casualty | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned	617.4	650.1	683.5
Net investment income	61.1	42.6	41.7
Net income	57.0	76.5	54.3
Assets	1,243.4	1,324.9	1,327.1
Supplemental | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned	125.3	130.7	65.8
Net investment income	24.8	17.8	7.5
Net income	46.3	43.1	18.0
Assets	854.9	811.5	747.6
Retirement | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned	38.5	29.7	29.1
Net investment income	255.9	229.8	245.5
Net income	52.0	20.1	(4.8)
Assets	9,939.1	9,198.7	8,330.1
Life | Operating Segments
Summarized financial information for these segments
Premiums and contract charges earned	108.4	120.2	119.6
Net investment income	83.1	69.8	72.0
Net income	16.1	10.4	17.6
Assets	2,129.5	2,044.5	1,965.0
Corporate & Other
Summarized financial information for these segments
Net investment income	(0.1)	(0.2)	(0.1)
Net income	(28.6)	(16.8)	99.3
Assets	$ 281.8	$ 182.3	$ 173.0