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Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,390.0	$ 2,435.4
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,352.1	1,389.6
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	917.8	937.2
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	91.5	75.0
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	28.6	33.6
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,426.4	1,104.2
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	341.2	374.7
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	168.2	343.5
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	353.3	169.3
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	87.8	355.6
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	13.1	88.1
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	1,432.6	1,108.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	342.6	376.5
Financing Receivable, Originated Three Years before Latest Fiscal Year	168.8	344.8
Financing Receivable, Originated Two Years before Latest Fiscal Year	354.4	169.9
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	88.4	356.4
Financing Receivable, Originated in Current Fiscal Year	13.2	88.6
Financing Receivable, before Allowance for Credit Loss	2,400.0	2,444.7
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(10.0)	(9.3)
Commercial Real Estate \| Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	932.6	782.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	197.0	189.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	80.8	193.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	128.0	81.3
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	16.7	128.9
Financing Receivable, Originated in Current Fiscal Year	0.0	16.9
Financing Receivable, before Allowance for Credit Loss	1,355.1	1,392.6
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(3.0)	(3.0)	$ (2.4)	$ (2.6)
Commercial Real Estate \| Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	386.6	230.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	145.6	181.6
Financing Receivable, Originated Three Years before Latest Fiscal Year	79.6	150.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	226.4	80.1
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	71.7	227.5
Financing Receivable, Originated in Current Fiscal Year	13.2	71.7
Financing Receivable, before Allowance for Credit Loss	923.1	942.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(5.3)	(4.7)	(4.5)	(4.7)
Commercial Real Estate \| Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	84.3	67.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	8.4	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0	8.5
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	92.7	76.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(1.2)	(1.1)	(0.7)	(0.7)
Commercial Real Estate \| Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	29.1	28.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0	5.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	29.1	34.1
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(0.5)	(0.5)	$ (0.3)	$ (0.3)
Commercial Real Estate \| Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(6.2)	(4.3)
Commercial Real Estate \| Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(1.4)	(1.8)
Commercial Real Estate \| Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(0.6)	(1.3)
Commercial Real Estate \| Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(1.1)	(0.6)
Commercial Real Estate \| Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(0.6)	(0.8)
Commercial Real Estate \| Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	(0.1)	(0.5)
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	80.2	92.3
AA Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	69.3	53.8
Financing Receivable, Originated Four Years before Latest Fiscal Year	11.0	27.5
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0	11.1
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	80.3	92.4
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	803.7	843.9
A Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	569.6	485.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	96.0	123.3
Financing Receivable, Originated Three Years before Latest Fiscal Year	35.7	96.8
Financing Receivable, Originated Two Years before Latest Fiscal Year	80.3	35.9
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	23.8	80.6
Financing Receivable, Originated in Current Fiscal Year	0.0	24.0
Financing Receivable, before Allowance for Credit Loss	805.4	845.6
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,465.2	1,458.0
BBB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	752.4	534.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	235.6	219.8
Financing Receivable, Originated Three Years before Latest Fiscal Year	133.1	236.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	274.1	134.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	64.6	275.8
Financing Receivable, Originated in Current Fiscal Year	13.2	64.6
Financing Receivable, before Allowance for Credit Loss	1,473.0	1,465.1
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	40.9	41.2
BB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	41.3	35.7
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0	5.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0.0	0.0
Financing Receivable, Originated in Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	$ 41.3	$ 41.6