Mortgage Loans, Allowance for Credit Losses Rollforward (Details) - Commercial Real Estate - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 14.6	$ 14.6
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	3.9
Current Period Provisions	0.0	1.1
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	3.9	3.9
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	7.9
Current Period Provisions	1.2	4.5
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	9.1	9.1
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	1.0
Current Period Provisions	(0.3)	0.2
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	0.7	0.7
Loan To Value Ratio Above 85 Percent
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance	0.4
Current Period Provisions	0.5	0.5
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	0.9	0.9
Loan to Value Ratio Total
Financing Receivable, Allowance for Credit Loss Rollforward, Mortgage Loans
Financing Receivable, Allowance for Credit Loss, Beginning Balance	13.2
Current Period Provisions	1.4	6.3
Write-offs	0.0	0.0
Recoveries	0.0	0.0
Financing Receivable, Allowance for Credit Loss, Ending Balance	$ 14.6	$ 14.6