Mortgage Loans, Allowance for Credit Losses Rollforward (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Current Period Provisions	$ 1.4	$ 1.1	$ 1.9	$ 1.8
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			10.2
Financing Receivable, Allowance for Credit Loss, Ending Balance	12.1		12.1		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	3.6	3.0	3.8	3.0	3.0
Current Period Provisions	0.1	0.3	(0.1)	0.3
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	3.7	3.3	3.7	3.3	3.8
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	3.4	5.3	3.8	4.7	4.7
Current Period Provisions	(0.1)	(0.5)	(0.5)	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	3.3	4.8	3.3	4.8	3.8
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	1.2	1.2	1.2	1.1	1.1
Current Period Provisions	0.5	(0.3)	0.5	(0.2)
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	0.9	1.7	0.9	1.2
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	2.5	0.5	1.4	0.5	0.5
Current Period Provisions	0.9	1.6	2.0	1.6
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	3.4	2.1	3.4	2.1	1.4
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	10.7	10.0	10.2	9.3	9.3
Current Period Provisions	1.4	1.1	1.9	1.8
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	$ 12.1	$ 11.1	$ 12.1	$ 11.1	$ 10.2

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

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