Mortgage Loans Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss		$ (15.9)	$ (16.1)
Mortgage Loans	[1]	2,109.5	2,224.5
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	[1]	1,701.2	1,639.6
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	[1]	164.3	367.6
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	[1]	162.6	152.3
Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	[1]	81.4	65.0
Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans		1,458.9	1,541.0
Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans		320.7	154.2
Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans		85.1	329.6
Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans		65.8	87.3
Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans		45.6	66.4
Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans		133.4	46.0
AA Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans		125.6	117.8
A Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans		1,053.4	1,099.1
BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans		804.0	915.5
BB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans		120.5	85.0
B Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans		6.0	7.1
Commercial Real Estate
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year		1,472.4	1,554.7
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year		321.6	154.7
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year		85.4	330.6
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year		66.2	87.6
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year		45.8	66.8
Financing Receivable, Year One, Originated, Current Fiscal Year		134.0	46.2
Financing Receivable, before Allowance for Credit Loss		2,125.4	2,240.6
Financing Receivable, Allowance for Credit Loss		(15.9)	(16.1)	$ (10.2)
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]	1,264.1	1,229.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	[1]	238.9	112.9
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	[1]	63.0	210.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	[1]	38.5	40.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	[1]	11.6	38.7
Financing Receivable, Year One, Originated, Current Fiscal Year	[1]	89.4	11.7
Financing Receivable, before Allowance for Credit Loss	[1]	1,705.5	1,643.7
Financing Receivable, Allowance for Credit Loss	[1]	(4.3)	(4.2)	(3.8)
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	[1]	59.8	154.1
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	[1]	10.7	33.7
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	[1]	8.4	72.1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	[1]	27.7	46.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	[1]	34.2	28.1
Financing Receivable, Year One, Originated, Current Fiscal Year	[1]	24.2	34.5
Financing Receivable, before Allowance for Credit Loss	[1]	165.0	369.3
Financing Receivable, Allowance for Credit Loss	[1]	(0.7)	(1.7)	(3.8)
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	[1]	115.5	126.4
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	[1]	35.0	8.1
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	[1]	14.0	20.1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	[1]	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	[1]	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	[1]	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	[1]	164.5	154.6
Financing Receivable, Allowance for Credit Loss	[1]	(1.9)	(2.2)	(1.2)
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	[1]	33.0	44.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	[1]	37.0	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	[1]	0.0	28.4
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	[1]	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	[1]	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	[1]	20.4	0.0
Financing Receivable, before Allowance for Credit Loss	[1]	90.4	73.0
Financing Receivable, Allowance for Credit Loss	[1]	(9.0)	(8.0)	$ (1.4)
Commercial Real Estate | Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss		(13.5)	(13.7)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss		(0.9)	(0.5)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss		(0.3)	(1.0)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss		(0.4)	(0.3)
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss		(0.2)	(0.4)
Commercial Real Estate | Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss		(0.6)	(0.2)
Commercial Real Estate | AA Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year		100.1	111.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year		25.6	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year		0.0	6.4
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		125.7	117.9
Commercial Real Estate | A Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year		794.0	780.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year		144.9	99.6
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year		23.6	169.1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year		9.5	24.6
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year		6.4	9.5
Financing Receivable, Year One, Originated, Current Fiscal Year		77.2	18.0
Financing Receivable, before Allowance for Credit Loss		1,055.6	1,101.3
Commercial Real Estate | BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year		482.4	561.7
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year		131.2	55.1
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year		61.8	155.1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year		56.7	63.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year		39.4	57.3
Financing Receivable, Year One, Originated, Current Fiscal Year		36.4	28.2
Financing Receivable, before Allowance for Credit Loss		807.9	920.4
Commercial Real Estate | BB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year		81.7	86.8
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year		19.9	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		20.4	0.0
Financing Receivable, before Allowance for Credit Loss		122.0	86.8
Commercial Real Estate | B Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year		14.2	14.2
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		$ 14.2	$ 14.2

[1] Loan-to-Value Ratio utilizes the most recent internal valuation of the property