Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions		Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	$ 2,172.5		$ 2,224.5
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		1,612.3		1,639.6
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		327.5		367.6
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		169.1		152.3
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		63.6		65.0
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	1,576.7		1,541.0
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	325.1		154.2
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	86.3		329.6
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	66.2		87.3
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	45.9		66.4
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	[1]	72.3		46.0
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	[1]	1,590.4		1,554.7
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	326.0		154.7
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	86.6		330.6
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	66.5		87.6
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	46.1		66.8
Financing Receivable, Originated in Current Fiscal Year	[1]	72.5		46.2
Financing Receivable, before Allowance for Credit Loss	[1]	2,188.1		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]	1,261.9		1,229.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	209.0		112.9
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	40.1		210.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	38.6		40.8
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	11.7		38.7
Financing Receivable, Originated in Current Fiscal Year	[1]	54.8		11.7
Financing Receivable, before Allowance for Credit Loss	[1]	1,616.1		1,643.7
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	3.8	$ 3.8	4.2	$ 3.7	$ 3.6	$ 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]	158.2		154.1
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	44.3		33.7
Financing Receivable, Originated Three Years before Latest Fiscal Year	[1]	46.5		72.1
Financing Receivable, Originated Two Years before Latest Fiscal Year	[1]	27.9		46.8
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	[1]	34.4		28.1
Financing Receivable, Originated in Current Fiscal Year	[1]	17.7		34.5
Financing Receivable, before Allowance for Credit Loss	[1]	329.0		369.3
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	1.5	1.7	1.7	3.3	3.4	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]	108.3		126.4
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	61.9		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]	170.2		154.6
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	1.1	3.1	2.2	1.7	1.2	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]	62.0		44.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	[1]	10.8		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]	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	[1]	72.8		73.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	[1]	9.2	$ 8.0	8.0	$ 3.4	$ 2.5	$ 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]	13.7		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]	0.3		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]	0.2		0.2
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		95.2		117.8
AA Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		89.0		111.5
Financing Receivable, Originated Four Years before Latest Fiscal Year		6.2		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		95.2		117.9
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		1,073.9		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		820.1		780.5
Financing Receivable, Originated Four Years before Latest Fiscal Year		166.7		99.6
Financing Receivable, Originated Three Years before Latest Fiscal Year		24.1		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		6.5		9.5
Financing Receivable, Originated in Current Fiscal Year		49.1		18.0
Financing Receivable, before Allowance for Credit Loss		1,076.0		1,101.3
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		885.9		915.5
BBB Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		554.6		561.7
Financing Receivable, Originated Four Years before Latest Fiscal Year		153.1		55.1
Financing Receivable, Originated Three Years before Latest Fiscal Year		62.5		155.1
Financing Receivable, Originated Two Years before Latest Fiscal Year		57.0		63.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year		39.6		57.3
Financing Receivable, Originated in Current Fiscal Year		23.4		28.2
Financing Receivable, before Allowance for Credit Loss		890.2		920.4
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		101.4		85.0
BB Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		103.3		86.8
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		103.3		86.8
B Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount		16.1		7.1
B Credit Rating | Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year		23.4		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		$ 23.4		$ 14.2

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