Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost	$ 20,324.5	$ 18,103.9	$ 16,513.9
Reinsurance recoverables	1,189.3	1,098.0
Accrued interest receivable	542.6	552.5
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator
2021	2,587.3
2020	2,033.6	2,019.8
2019	2,706.9	2,769.9
2018	2,523.5	2,817.2
2017	1,739.3	2,049.3
2016		1,509.3
Prior	4,804.1	4,276.1
Mortgage loans, Total amortized cost	16,394.7	15,441.6	15,017.2
Accrued interest receivable	60.7	60.0
Commercial mortgage loans \| A- and above
Financing Receivable, Credit Quality Indicator
2021	2,275.9
2020	1,722.7	1,807.6
2019	2,412.9	2,486.8
2018	2,383.3	2,464.7
2017	1,437.2	1,780.8
2016		1,417.8
Prior	4,334.2	3,697.0
Mortgage loans, Total amortized cost	14,566.2	13,654.7
Commercial mortgage loans \| BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2021	278.6
2020	305.6	149.1
2019	294.0	194.1
2018	131.4	352.5
2017	302.1	262.7
2016		75.8
Prior	380.0	499.7
Mortgage loans, Total amortized cost	1,691.7	1,533.9
Commercial mortgage loans \| BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2021	32.8
2020	5.3	23.7
2019		69.0
2016		9.1
Prior	55.4	43.9
Mortgage loans, Total amortized cost	93.5	145.7
Commercial mortgage loans \| B+ and below
Financing Receivable, Credit Quality Indicator
2020		39.4
2019		20.0
2018	8.8
2017		5.8
2016		6.6
Prior	34.5	35.5
Mortgage loans, Total amortized cost	43.3	107.3
Direct financing leases
Financing Receivable, Credit Quality Indicator
2021	94.2
2020	113.1	209.7
2019	25.3	29.6
2018	57.3	65.8
2017	32.1	37.6
2016		19.3
Prior	287.9	348.8
Mortgage loans, Total amortized cost	609.9	710.8
Accrued interest receivable	1.2	0.5
Direct financing leases \| A- and above
Financing Receivable, Credit Quality Indicator
2021	11.7
2020	41.8	43.9
2019	1.4	1.6
2018	39.4	42.6
2017	16.6	19.2
2016		15.3
Prior	235.6	202.9
Mortgage loans, Total amortized cost	346.5	325.5
Direct financing leases \| BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2021	30.2
2020	57.9	94.9
2019	22.0	5.5
2018	17.9	11.3
2017	15.5	18.4
2016		3.0
Prior	50.2	35.5
Mortgage loans, Total amortized cost	193.7	168.6
Direct financing leases \| BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2021	50.8
2020	13.4	13.3
2019	1.9
Prior	2.1	1.9
Mortgage loans, Total amortized cost	68.2	15.2
Direct financing leases \| B+ and below
Financing Receivable, Credit Quality Indicator
2021	1.5
2020		57.6
2019		22.5
2018		11.9
2016		1.0
Prior		108.5
Mortgage loans, Total amortized cost	1.5	201.5
Residential mortgage loans
Financing Receivable, Credit Quality Indicator
2021	2,039.1
2020	511.9	699.1
2019	156.2	338.7
2018	91.2	167.8
2017	103.2	166.5
2016		168.0
Prior	418.3	411.4
Mortgage loans, Total amortized cost	3,319.9	1,951.5	$ 1,496.7
Accrued interest receivable	16.7	2.0
Residential mortgage loans \| Performing
Financing Receivable, Credit Quality Indicator
2021	2,039.1
2020	510.1	699.1
2019	155.6	336.7
2018	91.2	167.0
2017	102.4	165.1
2016		167.6
Prior	415.6	398.7
Mortgage loans, Total amortized cost	3,314.0	1,934.2
Residential mortgage loans \| Non-performing
Financing Receivable, Credit Quality Indicator
2020	1.8
2019	0.6	2.0
2018		0.8
2017	0.8	1.4
2016		0.4
Prior	2.7	12.7
Mortgage loans, Total amortized cost	$ 5.9	$ 17.3
Mortgage loans, Days delinquent to be considered non-performing	90 days

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Amount excluding accrued interest of financing receivable originated five years prior to current fiscal year.

+ References

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Amount excluding accrued interest, before allowance for credit loss, of financing receivable.

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Amount excluding accrued interest of financing receivable originated more than five years prior to current fiscal year.

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Amount excluding accrued interest of financing receivable originated four years prior to current fiscal year.

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Amount excluding accrued interest of financing receivable originated three years prior to current fiscal year.

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Amount excluding accrued interest of financing receivable originated in current fiscal year.

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Amount excluding accrued interest of financing receivable originated two years prior to current fiscal year.

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Amount excluding accrued interest of financing receivable originated in fiscal year prior to current fiscal year.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Amount before valuation allowance recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

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