Segment Information - Summary of Segment Activity (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) segment	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			5
Number of operating segments | segment			4
Summarized financial information for these segments
Premiums and contract charges earned	$ 225.8	$ 225.4	$ 453.4	$ 461.7
Net investment income	109.2	80.4	204.7	162.7
Net income (loss)	46.7	30.5	86.0	49.0
Assets	14,190.1		14,190.1		$ 13,471.8
Intersegment eliminations
Summarized financial information for these segments
Net investment income	(0.5)	(0.6)	(1.1)	(1.1)
Assets	(80.5)		(80.5)		(90.1)
Property and Casualty
Summarized financial information for these segments
Premiums and contract charges earned	155.0	156.2	310.8	322.7
Property and Casualty | Operating segments
Summarized financial information for these segments
Net investment income	21.7	6.3	32.5	16.6
Net income (loss)	19.3	11.3	47.2	37.9
Assets	1,313.2		1,313.2		1,324.9
Supplemental
Summarized financial information for these segments
Premiums and contract charges earned	31.6	33.3	63.3	66.3
Assets	855.0		855.0		811.5
Supplemental | Operating segments
Summarized financial information for these segments
Net investment income	6.3	4.0	11.6	7.5
Net income (loss)	12.0	9.5	23.4	20.0
Retirement
Summarized financial information for these segments
Premiums and contract charges earned	9.2	6.7	17.8	14.1
Retirement | Operating segments
Summarized financial information for these segments
Net investment income	62.0	55.1	122.4	108.6
Net income (loss)	11.5	9.7	22.1	8.8
Assets	9,800.6		9,800.6		9,198.7
Life
Summarized financial information for these segments
Premiums and contract charges earned	30.0	29.2	61.5	58.6
Life | Operating segments
Summarized financial information for these segments
Net investment income	19.8	15.6	39.4	31.2
Net income (loss)	5.0	1.9	5.7	2.5
Assets	2,142.9		2,142.9		2,044.5
Corporate and Other
Summarized financial information for these segments
Net investment income	(0.1)	0.0	(0.1)	(0.1)
Net income (loss)	(1.1)	$ (1.9)	(12.4)	$ (20.2)
Assets	$ 158.9		$ 158.9		$ 182.3