Mortgage Loans Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	$ (16.1)	$ (10.2)
Mortgage Loans	2,224.5	2,318.2
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,639.6	1,409.9
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	367.6	707.0
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	152.3	136.5
Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	65.0	64.8
Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,541.0	1,317.6
Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	154.2	334.8
Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	329.6	164.9
Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	87.3	347.1
Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	66.4	87.0
Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	46.0	66.8
AA Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	117.8	85.2
A Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,099.1	942.5
BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	915.5	1,249.5
BB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	85.0	41.0
B Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	7.1	0.0
Commercial Real Estate
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,554.7	1,323.9
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	154.7	336.1
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	330.6	165.5
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	87.6	348.1
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	66.8	87.6
Financing Receivable, Year One, Originated, Current Fiscal Year	46.2	67.2
Financing Receivable, before Allowance for Credit Loss	2,240.6	2,328.4
Financing Receivable, Allowance for Credit Loss	(16.1)	(10.2)	$ (9.3)
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	1,229.6	908.3
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	112.9	197.7
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	210.0	116.4
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	40.8	145.2
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	38.7	16.2
Financing Receivable, Year One, Originated, Current Fiscal Year	11.7	30.0
Financing Receivable, before Allowance for Credit Loss	1,643.7	1,413.8
Financing Receivable, Allowance for Credit Loss	(4.2)	(3.8)	(3.0)
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	154.1	252.1
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	33.7	138.4
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	72.1	40.8
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	46.8	171.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	28.1	71.4
Financing Receivable, Year One, Originated, Current Fiscal Year	34.5	37.2
Financing Receivable, before Allowance for Credit Loss	369.3	710.9
Financing Receivable, Allowance for Credit Loss	(1.7)	(3.8)	(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	126.4	97.3
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	8.1	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	20.1	8.3
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	31.9
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	154.6	137.5
Financing Receivable, Allowance for Credit Loss	(2.2)	(1.2)	(1.1)
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	44.6	66.2
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	28.4	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	73.0	66.2
Financing Receivable, Allowance for Credit Loss	(8.0)	(1.4)	$ (0.5)
Commercial Real Estate | Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(13.7)	(6.3)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.5)	(1.3)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.0)	(0.6)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.3)	(1.0)
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.4)	(0.6)
Commercial Real Estate | Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.2)	(0.4)
Commercial Real Estate | AA Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	111.5	74.3
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0	11.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	6.4	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	117.9	85.3
Commercial Real Estate | A Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	780.5	589.0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	99.6	100.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	169.1	94.1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	24.6	100.1
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	9.5	23.2
Financing Receivable, Year One, Originated, Current Fiscal Year	18.0	38.0
Financing Receivable, before Allowance for Credit Loss	1,101.3	944.7
Commercial Real Estate | BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	561.7	618.4
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	55.1	224.8
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	155.1	71.4
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	63.0	248.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	57.3	64.4
Financing Receivable, Year One, Originated, Current Fiscal Year	28.2	29.2
Financing Receivable, before Allowance for Credit Loss	920.4	1,256.2
Commercial Real Estate | BB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	86.8	42.2
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	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	86.8	$ 42.2
Commercial Real Estate | B Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	14.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	$ 14.2