Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions		Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans		$ (15.3)	$ (15.9)
Mortgage Loans, Carrying Amount	[1]	2,066.3	2,109.5
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	1,703.4	1,701.2
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	151.3	164.3
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	130.8	162.6
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	80.8	81.4
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	1,718.6	1,458.9
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	84.6	320.7
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	65.6	85.1
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	45.5	65.8
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	133.0	45.6
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	19.0	133.4
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		128.5	125.6
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		1,015.7	1,053.4
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		819.3	804.0
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		97.0	120.5
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		5.8	6.0
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	[1]	1,732.4	1,472.4
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	84.9	321.6
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	65.9	85.4
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	45.7	66.2
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	133.6	45.8
Financing Receivable, Originated in Current Fiscal Year	[1]	19.1	134.0
Financing Receivable, before Allowance for Credit Loss	[1]	2,081.6	2,125.4
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(15.3)	(15.9)	$ (16.6)	$ (16.1)
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	[1]	1,486.8	1,264.1
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	62.5	238.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	38.4	63.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	11.6	38.5
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	89.1	11.6
Financing Receivable, Originated in Current Fiscal Year	[1]	19.1	89.4
Financing Receivable, before Allowance for Credit Loss	[1]	1,707.5	1,705.5
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(4.1)	(4.3)	(3.8)	(4.2)
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	[1]	57.8	59.8
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	8.4	10.7
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	27.5	8.4
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	34.1	27.7
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	24.1	34.2
Financing Receivable, Originated in Current Fiscal Year	[1]	0.0	24.2
Financing Receivable, before Allowance for Credit Loss	[1]	151.9	165.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(0.6)	(0.7)	(1.7)	(1.7)
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	[1]	118.2	115.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	14.0	35.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	0.0	14.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	0.0	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	0.0	0.0
Financing Receivable, Originated in Current Fiscal Year	[1]	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	[1]	132.2	164.5
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(1.4)	(1.9)	(3.1)	(2.2)
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	[1]	69.6	33.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	0.0	37.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	0.0	0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	0.0	0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	20.4	0.0
Financing Receivable, Originated in Current Fiscal Year	[1]	0.0	20.4
Financing Receivable, before Allowance for Credit Loss	[1]	90.0	90.4
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(9.2)	(9.0)	$ (8.0)	$ (8.0)
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	[1]	(13.8)	(13.5)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(0.3)	(0.9)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(0.3)	(0.3)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(0.2)	(0.4)
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(0.6)	(0.2)
Commercial Real Estate | Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	(0.1)	(0.6)
Commercial Real Estate | AA Credit Rating
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		128.6	100.1
Financing Receivable, Originated Four Years before Latest Fiscal Year		0.0	25.6
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		128.6	125.7
Commercial Real Estate | A Credit Rating
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		894.7	794.0
Financing Receivable, Originated Four Years before Latest Fiscal Year		23.3	144.9
Financing Receivable, Originated Three Years before Latest Fiscal Year		9.4	23.6
Financing Receivable, Originated Two Years before Latest Fiscal Year		6.4	9.5
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		76.9	6.4
Financing Receivable, Originated in Current Fiscal Year		6.8	77.2
Financing Receivable, before Allowance for Credit Loss		1,017.5	1,055.6
Commercial Real Estate | BBB Credit Rating
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		617.0	482.4
Financing Receivable, Originated Four Years before Latest Fiscal Year		61.6	131.2
Financing Receivable, Originated Three Years before Latest Fiscal Year		56.5	61.8
Financing Receivable, Originated Two Years before Latest Fiscal Year		39.3	56.7
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		36.3	39.4
Financing Receivable, Originated in Current Fiscal Year		12.3	36.4
Financing Receivable, before Allowance for Credit Loss		823.0	807.9
Commercial Real Estate | BB Credit Rating
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		77.9	81.7
Financing Receivable, Originated Four Years before Latest Fiscal Year		0.0	19.9
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		20.4	0.0
Financing Receivable, Originated in Current Fiscal Year		0.0	20.4
Financing Receivable, before Allowance for Credit Loss		98.3	122.0
Commercial Real Estate | B Credit Rating
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		14.2	14.2
Financing Receivable, Originated Four Years before Latest Fiscal Year		0.0	0.0
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		$ 14.2	$ 14.2

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