Mortgage Loans Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	$ (9.3)
Mortgage Loans	2,435.4	$ 2,560.4
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,389.6	1,346.1
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	937.2	1,076.8
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	75.0	114.9
Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	33.6	22.6
Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,104.2	981.5
Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	374.7	304.1
Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	343.5	383.0
Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	169.3	359.4
Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	355.6	180.0
Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	88.1	352.4
AA Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	92.3	27.3
A Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	843.9	709.6
BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,458.0	1,802.6
BB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	41.2	20.9
Commercial Real Estate
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,108.5	984.1
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	376.5	305.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	344.8	384.4
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	169.9	360.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	356.4	180.7
Financing Receivable, Year One, Originated, Current Fiscal Year	88.6	353.2
Financing Receivable, before Allowance for Credit Loss	2,444.7	2,568.7
Financing Receivable, Allowance for Credit Loss	(9.3)	(8.3)
Commercial Real Estate | Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	782.6	779.1
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	189.0	146.9
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	193.9	163.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	81.3	80.7
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	128.9	54.3
Financing Receivable, Year One, Originated, Current Fiscal Year	16.9	124.7
Financing Receivable, before Allowance for Credit Loss	1,392.6	1,348.7
Financing Receivable, Allowance for Credit Loss	(3.0)	(2.6)	$ (3.4)
Commercial Real Estate | Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	230.2	115.7
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	181.6	115.4
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	150.9	215.4
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	80.1	280.1
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	227.5	126.4
Financing Receivable, Year One, Originated, Current Fiscal Year	71.7	228.5
Financing Receivable, before Allowance for Credit Loss	942.0	1,081.5
Financing Receivable, Allowance for Credit Loss	(4.7)	(4.7)	(7.3)
Commercial Real Estate | Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	67.5	89.3
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0	26.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	8.5	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	76.0	115.6
Financing Receivable, Allowance for Credit Loss	(1.1)	(0.7)	(1.3)
Commercial Real Estate | Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	28.2	0.0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	5.9	16.9
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0	6.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	34.1	22.9
Financing Receivable, Allowance for Credit Loss	(0.5)	(0.3)	$ (1.1)
Commercial Real Estate | Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(4.3)	(2.6)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.8)	(1.4)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.3)	(1.4)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.6)	(1.4)
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.8)	(0.7)
Commercial Real Estate | Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.5)	(0.8)
Commercial Real Estate | AA Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	53.8	3.3
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	27.5	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	11.1	24.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	92.4	27.3
Commercial Real Estate | A Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	485.0	414.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	123.3	68.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	96.8	71.1
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	35.9	28.9
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	80.6	17.6
Financing Receivable, Year One, Originated, Current Fiscal Year	24.0	110.6
Financing Receivable, before Allowance for Credit Loss	845.6	710.8
Commercial Real Estate | BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	534.0	561.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	219.8	227.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	236.9	283.3
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	134.0	331.9
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	275.8	163.1
Financing Receivable, Year One, Originated, Current Fiscal Year	64.6	242.6
Financing Receivable, before Allowance for Credit Loss	1,465.1	1,809.4
Commercial Real Estate | BB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	35.7	5.0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	5.9	10.2
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0	6.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0	0.0
Financing Receivable, before Allowance for Credit Loss	$ 41.6	$ 21.2