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Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	$ 2,372.8		$ 2,435.4
Loan to Value Ratio Below or Equal to 65 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,381.2		1,389.6
Loan To Value Ratio Above 65 To 75 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	815.2		937.2
Loan To Value Ratio Above 75 To 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	111.5		75.0
Loan To Value Ratio Above 85 Percent
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	64.9		33.6
Originated Five or More Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,399.0		1,104.2
Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	339.2		374.7
Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	167.1		343.5
Originated Two Years before Last Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	351.3		169.3
Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	87.7		355.6
Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	28.5		88.1
Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	1,406.6		1,108.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	340.5		376.5
Financing Receivable, Originated Three Years before Latest Fiscal Year	167.7		344.8
Financing Receivable, Originated Two Years before Latest Fiscal Year	352.3		169.9
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	88.2		356.4
Financing Receivable, Originated in Current Fiscal Year	28.6		88.6
Financing Receivable, before Allowance for Credit Loss	2,383.9		2,444.7
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	11.1		9.3
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	951.5		782.6
Financing Receivable, Originated Four Years before Latest Fiscal Year	200.4		189.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	80.2		193.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	135.9		81.3
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	16.6		128.9
Financing Receivable, Originated in Current Fiscal Year	0.0		16.9
Financing Receivable, before Allowance for Credit Loss	1,384.6		1,392.6
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	3.3	$ 3.0	3.0	$ 2.3	$ 2.4	$ 2.6
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	284.1		230.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	140.1		181.6
Financing Receivable, Originated Three Years before Latest Fiscal Year	79.1		150.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	216.4		80.1
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	71.6		227.5
Financing Receivable, Originated in Current Fiscal Year	28.6		71.7
Financing Receivable, before Allowance for Credit Loss	819.9		942.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	4.8	5.3	4.7	4.8	4.5	4.7
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	104.0		67.5
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0		0.0
Financing Receivable, Originated Three Years before Latest Fiscal Year	8.4		0.0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0.0		8.5
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	112.4		76.0
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.9	1.2	1.1	0.9	0.7	0.7
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	67.0		28.2
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0		5.9
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	0.0		0.0
Financing Receivable, Originated in Current Fiscal Year	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	67.0		34.1
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	2.1	$ 0.5	0.5	$ 0.4	$ 0.3	$ 0.3
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	7.6		4.3
Commercial Real Estate \| Originated Four Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	1.3		1.8
Commercial Real Estate \| Originated Three Years before Last Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.6		1.3
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		0.6
Commercial Real Estate \| Originated in Fiscal Year before Latest Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.5		0.8
Commercial Real Estate \| Originated in Current Fiscal Year
Mortgage Loans on Real Estate
Financing Receivable, Allowance for Credit Losses, Mortgage Loans	0.1		0.5
AA Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	79.6		92.3
AA Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	68.7		53.8
Financing Receivable, Originated Four Years before Latest Fiscal Year	11.0		27.5
Financing Receivable, Originated Three Years before Latest Fiscal Year	0.0		11.1
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	79.7		92.4
A Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	826.6		843.9
A Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	565.5		485.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	95.3		123.3
Financing Receivable, Originated Three Years before Latest Fiscal Year	48.5		96.8
Financing Receivable, Originated Two Years before Latest Fiscal Year	80.0		35.9
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	23.6		80.6
Financing Receivable, Originated in Current Fiscal Year	15.5		24.0
Financing Receivable, before Allowance for Credit Loss	828.4		845.6
BBB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	1,404.0		1,458.0
BBB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	707.8		534.0
Financing Receivable, Originated Four Years before Latest Fiscal Year	234.2		219.8
Financing Receivable, Originated Three Years before Latest Fiscal Year	119.2		236.9
Financing Receivable, Originated Two Years before Latest Fiscal Year	272.3		134.0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	64.6		275.8
Financing Receivable, Originated in Current Fiscal Year	13.1		64.6
Financing Receivable, before Allowance for Credit Loss	1,411.2		1,465.1
BB Credit Rating
Mortgage Loans on Real Estate
Mortgage Loans, Carrying Amount	62.6		41.2
BB Credit Rating \| Commercial Real Estate
Mortgage Loans on Real Estate
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	64.6		35.7
Financing Receivable, Originated Four Years before Latest Fiscal Year	0.0		5.9
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	0.0		0.0
Financing Receivable, Originated in Current Fiscal Year	0.0		0.0
Financing Receivable, before Allowance for Credit Loss	$ 64.6		$ 41.6