Mortgage Loans Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	$ (8.3)	$ (13.1)
Mortgage Loans	2,560.4	2,432.1
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,346.1	1,189.4
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,076.8	1,000.3
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	114.9	155.8
Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	22.6	86.6
Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	981.5	769.6
Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	304.1	405.3
Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	383.0	318.8
Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	359.4	389.6
Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	180.0	365.9
Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	352.4	182.9
AA Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	27.3	3.5
A Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	709.6	510.0
BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,802.6	1,863.0
BB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	20.9	39.4
B Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	0.0	16.2
Commercial Real Estate
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	984.1	772.0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	305.5	407.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	384.4	320.7
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	360.8	392.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	180.7	368.8
Financing Receivable, Year One, Originated, Current Fiscal Year	353.2	184.4
Financing Receivable, before Allowance for Credit Loss	2,568.7	2,445.2
Financing Receivable, Allowance for Credit Loss	(8.3)	(13.1)
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	779.1	598.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	146.9	257.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	163.0	139.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	80.7	77.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	54.3	82.8
Financing Receivable, Year One, Originated, Current Fiscal Year	124.7	37.2
Financing Receivable, before Allowance for Credit Loss	1,348.7	1,192.8
Financing Receivable, Allowance for Credit Loss	(2.6)	(3.4)	$ (2.8)
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	115.7	47.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	115.4	122.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	215.4	109.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	280.1	294.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	126.4	286.0
Financing Receivable, Year One, Originated, Current Fiscal Year	228.5	147.2
Financing Receivable, before Allowance for Credit Loss	1,081.5	1,007.6
Financing Receivable, Allowance for Credit Loss	(4.7)	(7.3)	(4.6)
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	89.3	78.7
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	26.3	27.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0	37.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	13.3
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	115.6	157.2
Financing Receivable, Allowance for Credit Loss	(0.7)	(1.3)	(0.5)
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	0.0	47.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	16.9	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	6.0	33.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0	6.1
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	22.9	87.6
Financing Receivable, Allowance for Credit Loss	(0.3)	(1.1)	$ (0.4)
Commercial Real Estate | Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(2.6)	(2.4)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.4)	(2.0)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.4)	(1.9)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.4)	(2.4)
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.7)	(2.9)
Commercial Real Estate | Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(0.8)	(1.5)
Commercial Real Estate | AA Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	3.3	3.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	24.0	0.0
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	27.3	3.5
Commercial Real Estate | A Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	414.6	240.3
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	68.0	119.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	71.1	56.3
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	28.9	60.2
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	17.6	16.9
Financing Receivable, Year One, Originated, Current Fiscal Year	110.6	18.0
Financing Receivable, before Allowance for Credit Loss	710.8	511.2
Commercial Real Estate | BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	561.2	482.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	227.3	287.8
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	283.3	253.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	331.9	331.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	163.1	351.9
Financing Receivable, Year One, Originated, Current Fiscal Year	242.6	166.4
Financing Receivable, before Allowance for Credit Loss	1,809.4	1,874.4
Commercial Real Estate | BB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	5.0	29.4
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	10.2	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	6.0	10.5
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	21.2	39.9
Commercial Real Estate | B Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	0.0	16.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0	0.0
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	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	$ 0.0	$ 16.2