Mortgage Loans, Allowance for Credit Losses Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Write-offs	$ 0.0	$ 0.0
Commercial Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Beginning Balance	10.2	9.3
Current Period Provisions	0.5	0.7
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	10.7	10.0
Commercial Real Estate | Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Beginning Balance	3.8	3.0
Current Period Provisions	(0.2)	0.0
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	3.6	3.0
Commercial Real Estate | Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Beginning Balance	3.8	4.7
Current Period Provisions	(0.4)	0.6
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	3.4	5.3
Commercial Real Estate | Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Beginning Balance	1.2	1.1
Current Period Provisions	0.0	0.1
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	1.2	1.2
Commercial Real Estate | Loan To Value Ratio Above 85 Percent
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Beginning Balance	1.4	0.5
Current Period Provisions	1.1	0.0
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 2.5	$ 0.5