Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions		Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans		$ 15.6		$ 16.1
Mortgage Loans, Carrying Amount	[1],[2]	2,129.8		2,224.5
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	1,648.1		1,639.6
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	252.7		367.6
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	132.6		152.3
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[2]	96.4		65.0
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	1,517.3		1,541.0
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	322.9		154.2
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	85.7		329.6
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	65.9		87.3
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	45.7		66.4
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	92.3		46.0
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	[1]	1,530.7		1,554.7
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	323.8		154.7
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	86.0		330.6
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	66.3		87.6
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	45.9		66.8
Financing Receivable, Originated in Current Fiscal Year	[1]	92.7		46.2
Financing Receivable, before Allowance for Credit Loss	[1]	2,145.4		2,240.6
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	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	[1]	1,267.1		1,229.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	240.6		112.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	39.7		210.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	38.5		40.8
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	11.6		38.7
Financing Receivable, Originated in Current Fiscal Year	[1]	54.7		11.7
Financing Receivable, before Allowance for Credit Loss	[1]	1,652.2		1,643.7
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	4.1	$ 3.8	4.2	$ 4.0	$ 3.7	$ 3.8
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]	117.0		154.1
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	10.8		33.7
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	46.3		72.1
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	27.8		46.8
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	34.3		28.1
Financing Receivable, Originated in Current Fiscal Year	[1]	17.6		34.5
Financing Receivable, before Allowance for Credit Loss	[1]	253.8		369.3
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	1.1	1.5	1.7	2.2	3.3	3.8
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]	98.7		126.4
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	35.1		8.1
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	0.0		20.1
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]	133.8		154.6
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	1.2	1.1	2.2	2.5	1.7	1.2
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]	47.9		44.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	37.3		0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	0.0		28.4
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]	20.4		0.0
Financing Receivable, before Allowance for Credit Loss	[1]	105.6		73.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	9.2	$ 9.2	8.0	$ 7.8	$ 3.4	$ 1.4
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]	13.4		13.7
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	0.9		0.5
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		1.0
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	0.4		0.3
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]	0.2		0.4
Commercial Real Estate | Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	0.4		0.2
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		105.0		117.8
AA Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		98.9		111.5
Financing Receivable, Originated Four Years before Latest Fiscal Year		6.1		0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year		0.0		6.4
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		105.0		117.9
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		1,058.7		1,099.1
A Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		806.5		780.5
Financing Receivable, Originated Four Years before Latest Fiscal Year		165.6		99.6
Financing Receivable, Originated Three Years before Latest Fiscal Year		23.8		169.1
Financing Receivable, Originated Two Years before Latest Fiscal Year		9.5		24.6
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		6.4		9.5
Financing Receivable, Originated in Current Fiscal Year		49.0		18.0
Financing Receivable, before Allowance for Credit Loss		1,060.8		1,101.3
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		840.6		915.5
BBB Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		520.2		561.7
Financing Receivable, Originated Four Years before Latest Fiscal Year		142.8		55.1
Financing Receivable, Originated Three Years before Latest Fiscal Year		62.2		155.1
Financing Receivable, Originated Two Years before Latest Fiscal Year		56.8		63.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		39.5		57.3
Financing Receivable, Originated in Current Fiscal Year		23.3		28.2
Financing Receivable, before Allowance for Credit Loss		844.8		920.4
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		110.5		85.0
BB Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		82.5		86.8
Financing Receivable, Originated Four Years before Latest Fiscal Year		9.3		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		20.4		0.0
Financing Receivable, before Allowance for Credit Loss		112.2		86.8
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		15.0		7.1
B Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		22.6		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		$ 22.6		$ 14.2

[1] 1Loan-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