Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions		Sep. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Loss		$ (16.4)		$ (15.9)
Mortgage Loans, Carrying Amount	[1]	2,053.2		2,109.5
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	1,661.8		1,701.2
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	216.2		164.3
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	122.4		162.6
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	52.8		81.4
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	1,594.3		1,458.9
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	83.5		320.7
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	65.2		85.1
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	45.3		65.8
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	132.2		45.6
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	132.7		133.4
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	[2]	1,609.0		1,472.4
Financing Receivable, Originated Four Years before Latest Fiscal Year	[2]	83.7		321.6
Financing Receivable, Originated Three Years before Latest Fiscal Year	[2]	65.5		85.4
Financing Receivable, Originated Two Years before Latest Fiscal Year	[2]	45.5		66.2
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[2]	132.7		45.8
Financing Receivable, Originated in Current Fiscal Year	[2]	133.2		134.0
Financing Receivable, before Allowance for Credit Loss	[2]	2,069.6		2,125.4
Financing Receivable, Allowance for Credit Loss	[2]	(16.4)	$ (16.0)	(15.9)	$ (15.6)	$ (15.6)	$ (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	[2]	1,359.6		1,264.1
Financing Receivable, Originated Four Years before Latest Fiscal Year	[2]	61.6		238.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	[2]	31.8		63.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[2]	11.6		38.5
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[2]	95.0		11.6
Financing Receivable, Originated in Current Fiscal Year	[2]	106.5		89.4
Financing Receivable, before Allowance for Credit Loss	[2]	1,666.1		1,705.5
Financing Receivable, Allowance for Credit Loss	[2]	(4.3)	(4.1)	(4.3)	(4.1)	(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	[2]	97.4		59.8
Financing Receivable, Originated Four Years before Latest Fiscal Year	[2]	8.3		10.7
Financing Receivable, Originated Three Years before Latest Fiscal Year	[2]	33.7		8.4
Financing Receivable, Originated Two Years before Latest Fiscal Year	[2]	33.9		27.7
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[2]	17.3		34.2
Financing Receivable, Originated in Current Fiscal Year	[2]	26.7		24.2
Financing Receivable, before Allowance for Credit Loss	[2]	217.3		165.0
Financing Receivable, Allowance for Credit Loss	[2]	(1.1)	(0.8)	(0.7)	(1.1)	(1.5)	(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	[2]	110.4		115.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	[2]	13.8		35.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	[2]	0.0		14.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[2]	0.0		0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[2]	0.0		0.0
Financing Receivable, Originated in Current Fiscal Year	[2]	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	[2]	124.2		164.5
Financing Receivable, Allowance for Credit Loss	[2]	(1.8)	(1.0)	(1.9)	(1.2)	(1.1)	(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	[2]	41.6		33.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	[2]	0.0		37.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	[2]	0.0		0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[2]	0.0		0.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[2]	20.4		0.0
Financing Receivable, Originated in Current Fiscal Year	[2]	0.0		20.4
Financing Receivable, before Allowance for Credit Loss	[2]	62.0		90.4
Financing Receivable, Allowance for Credit Loss	[2]	(9.2)	$ (10.1)	(9.0)	$ (9.2)	$ (9.2)	$ (8.0)
Commercial Real Estate | Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Loss	[2]	(14.7)		(13.5)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Loss	[2]	(0.2)		(0.9)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Loss	[2]	(0.3)		(0.3)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Loss	[2]	(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 Loss	[2]	(0.5)		(0.2)
Commercial Real Estate | Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Loss	[2]	(0.5)		(0.6)
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		124.5		125.6
AA Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		124.6		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		124.6		125.7
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		1,038.8		1,053.4
A Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		844.1		794.0
Financing Receivable, Originated Four Years before Latest Fiscal Year		22.7		144.9
Financing Receivable, Originated Three Years before Latest Fiscal Year		24.4		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		76.2		6.4
Financing Receivable, Originated in Current Fiscal Year		67.2		77.2
Financing Receivable, before Allowance for Credit Loss		1,041.0		1,055.6
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		761.1		804.0
BBB Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		521.6		482.4
Financing Receivable, Originated Four Years before Latest Fiscal Year		61.0		131.2
Financing Receivable, Originated Three Years before Latest Fiscal Year		41.1		61.8
Financing Receivable, Originated Two Years before Latest Fiscal Year		39.1		56.7
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		36.1		39.4
Financing Receivable, Originated in Current Fiscal Year		66.0		36.4
Financing Receivable, before Allowance for Credit Loss		764.9		807.9
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		123.2		120.5
BB Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		104.5		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		124.9		122.0
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		5.6		6.0
B Credit Rating | Commercial Real Estate
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] Loan-to-Value Ratio utilizes the most recent internal valuation of the property

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