Investments - Credit Quality Indicators (Details) - Commercial Portfolio Segment $ in Millions	Sep. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Greater than 2.0
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 0.0	$ 24.2
Financing receivable, originated year one	25.3	11.7
Financing receivable, originated year two	11.6	0.0
Financing receivable, originated year three	0.0	0.0
Financing receivable, originated year four	0.0	0.0
Prior	47.5	50.8
Total	$ 84.4	$ 86.7
% of Total	0.258	0.292
1.5 to 2.0
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 16.9	$ 26.8
Financing receivable, originated year one	26.4	11.6
Financing receivable, originated year two	11.4	0.0
Financing receivable, originated year three	0.0	0.0
Financing receivable, originated year four	0.0	4.6
Prior	10.6	6.6
Total	$ 65.3	$ 49.6
% of Total	0.200	0.167
1.0 to 1.5
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 24.7	$ 25.7
Financing receivable, originated year one	25.7	63.0
Financing receivable, originated year two	56.5	0.0
Financing receivable, originated year three	0.0	0.0
Financing receivable, originated year four	0.0	0.0
Prior	12.4	13.7
Total	$ 119.3	$ 102.4
% of Total	0.366	0.344
Less than 1.0
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 0.0	$ 0.0
Financing receivable, originated year one	0.0	49.2
Financing receivable, originated year two	49.6	2.7
Financing receivable, originated year three	2.8	0.0
Financing receivable, originated year four	0.0	0.0
Prior	5.0	6.8
Total	$ 57.4	$ 58.7
% of Total	0.176	0.197
Total
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 41.6	$ 76.7
Financing receivable, originated year one	77.4	135.5
Financing receivable, originated year two	129.1	2.7
Financing receivable, originated year three	2.8	0.0
Financing receivable, originated year four	0.0	4.6
Prior	75.5	77.9
Total	$ 326.4	$ 297.4
% of Total	1.000	1.000
70% and less
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 39.1	$ 44.0
Financing receivable, originated year one	47.7	45.1
Financing receivable, originated year two	42.9	0.0
Financing receivable, originated year three	0.0	0.0
Financing receivable, originated year four	0.0	0.0
Prior	69.2	76.0
Total	$ 198.9	$ 165.1
% of Total	0.610	0.555
71% to 80%
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 2.5	$ 32.7
Financing receivable, originated year one	29.7	75.7
Financing receivable, originated year two	68.6	2.7
Financing receivable, originated year three	2.8	0.0
Financing receivable, originated year four	0.0	4.6
Prior	4.5	0.0
Total	$ 108.1	$ 115.7
% of Total	0.331	0.389
81% to 95%
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 0.0	$ 0.0
Financing receivable, originated year one	0.0	14.7
Financing receivable, originated year two	17.6	0.0
Financing receivable, originated year three	0.0	0.0
Financing receivable, originated year four	0.0	0.0
Prior	1.8	0.0
Total	$ 19.4	$ 14.7
% of Total	0.059	0.050
Greater than 95%
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 0.0	$ 0.0
Financing receivable, originated year one	0.0	0.0
Financing receivable, originated year two	0.0	0.0
Financing receivable, originated year three	0.0	0.0
Financing receivable, originated year four	0.0	0.0
Prior	0.0	1.9
Total	$ 0.0	$ 1.9
% of Total	0	0.006
Total
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated current year	$ 41.6	$ 76.7
Financing receivable, originated year one	77.4	135.5
Financing receivable, originated year two	129.1	2.7
Financing receivable, originated year three	2.8	0.0
Financing receivable, originated year four	0.0	4.6
Prior	75.5	77.9
Total	$ 326.4	$ 297.4
% of Total	1.000	1.000