Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost	$ 21,281.3	$ 20,324.5
Reinsurance recoverables	1,115.3	1,189.3
Accrued interest receivable	573.5	542.6
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator
2022	503.2
2021	2,647.2	2,587.3
2020	2,117.5	2,033.6
2019	2,706.5	2,706.9
2018	2,527.2	2,523.5
2017		1,739.3
Prior	6,358.6	4,804.1
Mortgage loans, Total amortized cost	16,860.2	16,394.7
Accrued interest receivable	63.4	60.7
Commercial mortgage loans | A- and above
Financing Receivable, Credit Quality Indicator
2022	491.7
2021	2,282.4	2,275.9
2020	1,800.0	1,722.7
2019	2,410.7	2,412.9
2018	2,418.7	2,383.3
2017		1,437.2
Prior	5,577.2	4,334.2
Mortgage loans, Total amortized cost	14,980.7	14,566.2
Commercial mortgage loans | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2022	11.5
2021	324.2	278.6
2020	311.6	305.6
2019	295.8	294.0
2018	99.7	131.4
2017		302.1
Prior	700.5	380.0
Mortgage loans, Total amortized cost	1,743.3	1,691.7
Commercial mortgage loans | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2021	40.6	32.8
2020	5.9	5.3
Prior	46.0	55.4
Mortgage loans, Total amortized cost	92.5	93.5
Commercial mortgage loans | B+ and below
Financing Receivable, Credit Quality Indicator
2018	8.8	8.8
Prior	34.9	34.5
Mortgage loans, Total amortized cost	43.7	43.3
Direct financing leases
Financing Receivable, Credit Quality Indicator
2022	89.1
2021	93.5	94.2
2020	124.4	113.1
2019	27.6	25.3
2018	54.9	57.3
2017		32.1
Prior	281.2	287.9
Mortgage loans, Total amortized cost	670.7	609.9
Accrued interest receivable	0.0	1.2
Direct financing leases | A- and above
Financing Receivable, Credit Quality Indicator
2022	75.2
2021	13.0	11.7
2020	45.9	41.8
2019	1.5	1.4
2018	43.3	39.4
2017		16.6
Prior	207.0	235.6
Mortgage loans, Total amortized cost	385.9	346.5
Direct financing leases | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2022	13.9
2021	22.5	30.2
2020	63.7	57.9
2019	24.1	22.0
2018	11.6	17.9
2017		15.5
Prior	72.0	50.2
Mortgage loans, Total amortized cost	207.8	193.7
Direct financing leases | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2021	56.4	50.8
2020	14.8	13.4
2019	2.0	1.9
Prior	2.2	2.1
Mortgage loans, Total amortized cost	75.4	68.2
Direct financing leases | B+ and below
Financing Receivable, Credit Quality Indicator
2021	1.6	1.5
Mortgage loans, Total amortized cost	1.6	1.5
Residential mortgage loans
Financing Receivable, Credit Quality Indicator
2022	443.3
2021	2,078.0	2,039.1
2020	497.2	511.9
2019	151.8	156.2
2018	84.0	91.2
2017		103.2
Prior	496.1	418.3
Mortgage loans, Total amortized cost	3,750.4	3,319.9
Accrued interest receivable	18.8	16.7
Residential mortgage loans | Performing
Financing Receivable, Credit Quality Indicator
2022	443.3
2021	2,077.4	2,039.1
2020	494.1	510.1
2019	149.7	155.6
2018	83.2	91.2
2017		102.4
Prior	491.4	415.6
Mortgage loans, Total amortized cost	3,739.1	3,314.0
Residential mortgage loans | Non-performing
Financing Receivable, Credit Quality Indicator
2021	0.6
2020	3.1	1.8
2019	2.1	0.6
2018	0.8
2017		0.8
Prior	4.7	2.7
Mortgage loans, Total amortized cost	$ 11.3	$ 5.9
Mortgage loans, Days delinquent to be considered non-performing	90 days