Mortgage Loans Sorted by Applicable Credit Quality Indicators (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	$ (13.1)		$ 0.0
Mortgage Loans	2,432.1		2,397.0
Loan to Value Ratio Below or Equal to 65 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,189.4		1,215.1
Loan To Value Ratio Above 65 To 75 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,000.3		1,053.0
Loan To Value Ratio Above 75 To 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	155.8		91.4
Loan To Value Ratio Above 85 Percent
Financing Receivable, Credit Quality Indicator
Mortgage Loans	86.6		37.5
Originated Five or More Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	769.6
Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	405.3
Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	318.8
Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	389.6
Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	365.9
Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Mortgage Loans	182.9
AA Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	3.5		0.0
A Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	510.0		485.6
BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	1,863.0		1,911.4
BB Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	39.4		0.0
B Credit Rating
Financing Receivable, Credit Quality Indicator
Mortgage Loans	16.2		0.0
Commercial Real Estate
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	772.0
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	407.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	320.7
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	392.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	368.8
Financing Receivable, Year One, Originated, Current Fiscal Year	184.4
Financing Receivable, before Allowance for Credit Loss	2,445.2		$ 2,397.0
Financing Receivable, Allowance for Credit Loss	(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	598.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	257.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	139.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	77.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	82.8
Financing Receivable, Year One, Originated, Current Fiscal Year	37.2
Financing Receivable, before Allowance for Credit Loss	1,192.8
Financing Receivable, Allowance for Credit Loss	(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	47.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	122.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	109.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	294.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	286.0
Financing Receivable, Year One, Originated, Current Fiscal Year	147.2
Financing Receivable, before Allowance for Credit Loss	1,007.6
Financing Receivable, Allowance for Credit Loss	(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	78.7
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	27.3
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	37.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	13.3
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	157.2
Financing Receivable, Allowance for Credit Loss	(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	47.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	33.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	6.1
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	87.6
Financing Receivable, Allowance for Credit Loss	(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	(2.4)
Commercial Real Estate | Originated Four Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(2.0)
Commercial Real Estate | Originated Three Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.9)
Commercial Real Estate | Originated Two Years before Last Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(2.4)
Commercial Real Estate | Originated in Fiscal Year before Latest Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(2.9)
Commercial Real Estate | Originated in Current Fiscal Year
Financing Receivable, Credit Quality Indicator
Financing Receivable, Allowance for Credit Loss	(1.5)
Commercial Real Estate | AA Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	3.5
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	3.5
Commercial Real Estate | A Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	240.3
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	119.5
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	56.3
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	60.2
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	16.9
Financing Receivable, Year One, Originated, Current Fiscal Year	18.0
Financing Receivable, before Allowance for Credit Loss	511.2
Commercial Real Estate | BBB Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	482.6
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	287.8
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	253.9
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	331.8
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	351.9
Financing Receivable, Year One, Originated, Current Fiscal Year	166.4
Financing Receivable, before Allowance for Credit Loss	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	29.4
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	10.5
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	39.9
Commercial Real Estate | B Credit Rating
Financing Receivable, Credit Quality Indicator
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	16.2
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0.0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	0.0
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0.0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0.0
Financing Receivable, Year One, Originated, Current Fiscal Year	0.0
Financing Receivable, before Allowance for Credit Loss	$ 16.2