Segment Information - Summary of Segment Activity (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information
Number of reportable segments | segment			5
Number of operating segments | segment			4
Summarized financial information for these segments
Premiums and contract charges earned	$ 225.4	$ 235.3	$ 678.8	$ 697.0
Net investment income	103.7	93.7	308.4	256.4
Net income (loss)	16.3	36.5	102.3	85.5
Assets	14,265.0		14,265.0		$ 13,471.8
Intersegment eliminations
Summarized financial information for these segments
Net investment income	(0.6)	(0.6)	(1.7)	(1.7)
Assets	(69.2)		(69.2)		(90.1)
Property and Casualty
Summarized financial information for these segments
Premiums and contract charges earned	153.3	166.0	464.1	488.7
Property and Casualty | Operating segments
Summarized financial information for these segments
Net investment income	11.3	13.7	43.8	30.3
Net income (loss)	(4.7)	15.8	42.5	53.7
Assets	1,269.0		1,269.0		1,324.9
Supplemental
Summarized financial information for these segments
Premiums and contract charges earned	31.0	32.5	94.3	98.8
Assets	863.8		863.8		811.5
Supplemental | Operating segments
Summarized financial information for these segments
Net investment income	7.1	4.3	18.7	11.8
Net income (loss)	11.4	10.6	34.8	30.6
Retirement
Summarized financial information for these segments
Premiums and contract charges earned	9.9	7.4	27.7	21.5
Retirement | Operating segments
Summarized financial information for these segments
Net investment income	64.8	58.1	187.2	166.7
Net income (loss)	14.1	7.8	36.2	16.6
Assets	9,893.4		9,893.4		9,198.7
Life
Summarized financial information for these segments
Premiums and contract charges earned	31.2	29.4	92.7	88.0
Life | Operating segments
Summarized financial information for these segments
Net investment income	21.1	18.2	60.5	49.4
Net income (loss)	5.1	4.3	10.8	6.8
Assets	2,142.6		2,142.6		2,044.5
Corporate and Other
Summarized financial information for these segments
Net investment income	0.0	0.0	(0.1)	(0.1)
Net income (loss)	(9.6)	$ (2.0)	(22.0)	$ (22.2)
Assets	$ 165.4		$ 165.4		$ 182.3