Mortgage Loans Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	$ (10.2)
Mortgage Loans	2,318.2	$ 2,435.4
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,409.9	1,389.6
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	707.0	937.2
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	136.5	75.0
Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	64.8	33.6
Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,317.6	1,104.2
Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	334.8	374.7
Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	164.9	343.5
Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	347.1	169.3
Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	87.0	355.6
Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	66.8	88.1
AA Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	85.2	92.3
A Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	942.5	843.9
BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,249.5	1,458.0
BB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	41.0	41.2
Commercial Real Estate
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,323.9	1,108.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	336.1	376.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	165.5	344.8
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	348.1	169.9
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	87.6	356.4
Financing Receivable, Year One, Originated, Current Fiscal Year	67.2	88.6
Financing Receivable, before Allowance for Credit Loss	2,328.4	2,444.7
Financing Receivable, Allowance for Credit Loss	(10.2)	(9.3)	$ (8.3)
Commercial Real Estate | Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	908.3	782.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	197.7	189.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	116.4	193.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	145.2	81.3
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	16.2	128.9
Financing Receivable, Year One, Originated, Current Fiscal Year	30.0	16.9
Financing Receivable, before Allowance for Credit Loss	1,413.8	1,392.6
Financing Receivable, Allowance for Credit Loss	(3.8)	(3.0)	(2.6)
Commercial Real Estate | Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	252.1	230.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	138.4	181.6
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	40.8	150.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	171.0	80.1
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	71.4	227.5
Financing Receivable, Year One, Originated, Current Fiscal Year	37.2	71.7
Financing Receivable, before Allowance for Credit Loss	710.9	942.0
Financing Receivable, Allowance for Credit Loss	(3.8)	(4.7)	(4.7)
Commercial Real Estate | Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	97.3	67.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	8.3	0.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	31.9	8.5
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	137.5	76.0
Financing Receivable, Allowance for Credit Loss	(1.2)	(1.1)	(0.7)
Commercial Real Estate | Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	66.2	28.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0	5.9
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	66.2	34.1
Financing Receivable, Allowance for Credit Loss	(1.4)	(0.5)	$ (0.3)
Commercial Real Estate | Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(6.3)	(4.3)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.3)	(1.8)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.6)	(1.3)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.0)	(0.6)
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.6)	(0.8)
Commercial Real Estate | Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.4)	(0.5)
Commercial Real Estate | AA Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	74.3	53.8
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	11.0	27.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0	11.1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	85.3	92.4
Commercial Real Estate | A Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	589.0	485.0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	100.3	123.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	94.1	96.8
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	100.1	35.9
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	23.2	80.6
Financing Receivable, Year One, Originated, Current Fiscal Year	38.0	24.0
Financing Receivable, before Allowance for Credit Loss	944.7	845.6
Commercial Real Estate | BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	618.4	534.0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	224.8	219.8
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	71.4	236.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	248.0	134.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	64.4	275.8
Financing Receivable, Year One, Originated, Current Fiscal Year	29.2	64.6
Financing Receivable, before Allowance for Credit Loss	1,256.2	1,465.1
Commercial Real Estate | BB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	42.2	35.7
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0	5.9
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	$ 42.2	$ 41.6