Mortgage Loans, Allowance for Credit Losses Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Writeoff	$ 0.0	$ 0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	10.2
Commercial Real Estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss, Beginning Balance	9.3	8.3
Financing Receivable, Credit Loss, Expense (Reversal)	0.9	1.0
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	10.2	9.3
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	3.0	2.6
Financing Receivable, Credit Loss, Expense (Reversal)	0.8	0.4
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	3.8	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	4.7	4.7
Financing Receivable, Credit Loss, Expense (Reversal)	(0.9)	0.0
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	3.8	4.7
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	1.1	0.7
Financing Receivable, Credit Loss, Expense (Reversal)	0.1	0.4
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	1.2	1.1
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	0.5	0.3
Financing Receivable, Credit Loss, Expense (Reversal)	0.9	0.2
Financing Receivable, Allowance for Credit Loss, Writeoff	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Recovery	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 1.4	$ 0.5