Mortgage Loans, Allowance for Credit Losses Rollforward (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Current Period Provisions	$ (0.9)		$ 0.4	$ 0.9	$ 0.3
Write-offs		$ 0.0		0.0
Commercial Real Estate
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance		9.3		9.3
Financing Receivable, Allowance for Credit Loss, Ending Balance	10.2			10.2
Commercial Real Estate | Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance	3.3	3.0	2.3	3.0	2.6
Current Period Provisions	(0.1)		0.2	0.2	(0.1)
Write-offs	0.0		0.0	0.0	0.0
Recoveries	0.0		0.0	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	3.2		2.5	3.2	2.5
Commercial Real Estate | Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance	4.8	4.7	4.8	4.7	4.7
Current Period Provisions	(0.4)		0.4	(0.3)	0.5
Write-offs	0.0		0.0	0.0	0.0
Recoveries	0.0		0.0	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	4.4		5.2	4.4	5.2
Commercial Real Estate | Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance	0.9	1.1	0.9	1.1	0.7
Current Period Provisions	0.0		(0.2)	(0.2)	0.0
Write-offs	0.0		0.0	0.0	0.0
Recoveries	0.0		0.0	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	0.9		0.7	0.9	0.7
Commercial Real Estate | Loan To Value Ratio Above 85 Percent
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance	2.1	0.5	0.4	0.5	0.3
Current Period Provisions	(0.4)		(0.1)	1.2	0.0
Write-offs	0.0		0.0	0.0	0.0
Recoveries	0.0		0.0	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	1.7		0.3	1.7	0.3
Commercial Real Estate | Loan to Value Ratio Total
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance	11.1	$ 9.3	8.4	9.3	8.3
Current Period Provisions	(0.9)		0.3	0.9	0.4
Write-offs	0.0		0.0	0.0	0.0
Recoveries	0.0		0.0	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 10.2		$ 8.7	$ 10.2	$ 8.7