Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Vintage year, Direct financing leases
2024	$ 48.3
2023	1.8	$ 46.1
2022	126.3	145.5
2021	43.7	48.8
2020	90.0	111.6
2019		12.9
Prior		285.2
Prior	252.9
Direct financing leases, Total amortized cost	563.0	650.1
Reinsurance recoverable and deposit receivable	19,493.4	20,614.9
Accrued interest receivable on direct financing leases	1.2	1.2
Mortgage and other loans
Accrued interest receivable	647.3	619.2
A- and above
Vintage year, Direct financing leases
2024	1.0
2023		1.1
2022	38.5	122.0
2021	11.0	12.4
2020	33.8	56.7
2019		1.3
Prior		207.3
Prior	177.4
Direct financing leases, Total amortized cost	261.7	400.8
BBB+ thru BBB-
Vintage year, Direct financing leases
2024	4.9
2023	1.8	2.8
2022	87.3	23.5
2021	19.0	20.8
2020	52.8	46.9
2019		11.6
Prior		76.3
Prior	67.0
Direct financing leases, Total amortized cost	232.8	181.9
BB+ thru BB-
Vintage year, Direct financing leases
2024	38.8
2023		42.2
2022	0.5
2021	6.8	15.6
2020	3.4	8.0
Prior		1.6
Prior	8.5
Direct financing leases, Total amortized cost	58.0	67.4
B+ and below
Vintage year, Direct financing leases
2024	3.6
2021	6.9
Direct financing leases, Total amortized cost	10.5
Commercial mortgage loans
Vintage year, Mortgage loans
2024	1,607.8
2023	1,330.1	1,057.7
2022	1,620.2	1,775.3
2021	2,418.3	2,755.5
2020	1,686.6	1,802.0
2019		2,410.3
Prior	8,409.7	6,781.0
Mortgage loans, Total amortized cost	17,072.7	16,581.8
Mortgage and other loans
Accrued interest receivable	69.2	58.3
Commercial mortgage loans \| A- and above
Vintage year, Mortgage loans
2024	1,182.1
2023	793.1	664.8
2022	1,234.3	1,277.1
2021	2,101.3	2,364.0
2020	1,463.4	1,587.3
2019		1,954.4
Prior	6,594.0	5,576.6
Mortgage loans, Total amortized cost	13,368.2	13,424.2
Commercial mortgage loans \| BBB+ thru BBB-
Vintage year, Mortgage loans
2024	210.0
2023	393.6	282.4
2022	231.6	356.0
2021	269.1	374.8
2020	180.6	212.0
2019		337.3
Prior	1,217.5	911.7
Mortgage loans, Total amortized cost	2,502.4	2,474.2
Commercial mortgage loans \| BB+ thru BB-
Vintage year, Mortgage loans
2024	215.7
2023	143.4	110.5
2022	154.3	142.2
2021	47.9	16.7
2020	40.1	2.7
2019		35.2
Prior	271.7	234.2
Mortgage loans, Total amortized cost	873.1	541.5
Commercial mortgage loans \| B+ and below
Vintage year, Mortgage loans
2020	2.5
2019		83.4
Prior	326.5	58.5
Mortgage loans, Total amortized cost	329.0	141.9
Residential mortgage loans
Vintage year, Mortgage loans
2023		503.4
2022		1,068.3
2021		1,346.7
2020		229.7
2019		122.8
Prior		425.6
2024 mortgage loans excluding portfolio layer method basis adjustments	350.9
2023 mortgage loans excluding portfolio layer method basis adjustments	417.0
2022 mortgage loans excluding portfolio layer method basis adjustments	976.4
2021 mortgage loans excluding portfolio layer method basis adjustments	1,239.7
2020 mortgage loans excluding portfolio layer method basis adjustments	198.5
Prior mortgage loans excluding portfolio layer method basis adjustments	433.3
Total mortgage loans excluding portfolio layer method basis adjustments	3,615.8
Unallocated portfolio layer method basis adjustment	(8.4)
Mortgage loans, Total amortized cost	3,607.4	3,696.5
Mortgage and other loans
Accrued interest receivable	12.0	11.4
Residential mortgage loans \| Performing
Vintage year, Mortgage loans
2024	350.4
2023	411.5	502.3
2022	970.3	1,064.3
2021	1,234.9	1,341.8
2020	196.0	228.4
2019		120.8
Prior	427.6	421.0
Mortgage loans, Total amortized cost	3,590.7	3,678.6
Residential mortgage loans \| Non-performing
Vintage year, Mortgage loans
2024	0.5
2023	5.5	1.1
2022	6.1	4.0
2021	4.8	4.9
2020	2.5	1.3
2019		2.0
Prior	5.7	4.6
Mortgage loans, Total amortized cost	$ 25.1	17.9
Mortgage and other loans
Mortgage loans, delinquent to be considered non-performing (in days)	90 days
Other loans
Vintage year, Mortgage loans
2024	$ 84.3
2023	61.9	168.5
Prior	0.1	0.1
Mortgage loans, Total amortized cost	146.3	168.6
Mortgage and other loans
Accrued interest receivable	1.3	1.6
Other loans \| Performing
Vintage year, Mortgage loans
2024	84.2
2023	61.9	168.5
Mortgage loans, Total amortized cost	146.1	168.5
Other loans \| Non-performing
Vintage year, Mortgage loans
2024	0.1
Prior	0.1	0.1
Mortgage loans, Total amortized cost	$ 0.2	$ 0.1

The numbers of days for which a mortgage loan has been delinquent such that it is considered non-performing.

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Amount of net investment in direct financing lease originated five years prior to current fiscal year.

+ References

No definition available.

+ Details

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- Definition

Amount of net investment in direct financing lease originated six or more years prior to current fiscal year.

+ References

No definition available.

+ Details

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- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable before portfolio layer method adjustment. Excludes net investment in lease.

+ References

No definition available.

+ Details

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- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable of portfolio layer method adjustment. Excludes net investment in lease.

+ References

No definition available.

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- Definition

Amortized cost excluding accrued interest, of financing receivable before portfolio layer method basis adjustment originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

No definition available.

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- Definition

Amortized cost excluding accrued interest, of financing receivable before portfolio layer method basis adjustment originated in current fiscal year. Excludes net investment in lease.

+ References

No definition available.

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- Definition

Amortized cost excluding accrued interest of financing receivable before portfolio layer method basis adjustment originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

No definition available.

+ Details

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- Definition

Amortized cost excluding accrued interest of financing receivable before portfolio layer method basis adjustment originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

No definition available.

+ Details

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Amortized cost excluding accrued interest of financing receivable originated five years prior to current fiscal year. Excludes net investment in lease.

+ References

No definition available.

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- Definition

Amortized cost excluding accrued interest of financing receivable before portfolio layer method basis adjustment originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

No definition available.

+ Details

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- Definition

Amortized cost excluding accrued interest of financing receivable before portfolio layer method basis adjustment originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

No definition available.

+ Details

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- References

No definition available.

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- Definition

Amount excluding accrued interest, before allowance for credit loss, of net investment in direct financing lease.

+ References

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- Definition

Amount of net investment in direct financing lease originated more than five years prior to current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

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- Definition

Amount of net investment in direct financing lease originated four years prior to current fiscal year.

+ References

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- Definition

Amount of net investment in direct financing lease originated in current fiscal year.

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- Definition

Amount of net investment in direct financing lease originated in fiscal year prior to current fiscal year.

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- Definition

Amount of net investment in direct financing lease originated three years prior to current fiscal year.

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- Definition

Amount of net investment in direct financing lease originated two years prior to current fiscal year.

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- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

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- Definition

Amortized cost excluding accrued interest, of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

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Amortized cost excluding accrued interest, of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

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Amortized cost excluding accrued interest, of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

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Amortized cost excluding accrued interest, of financing receivable originated in current fiscal year. Excludes net investment in lease.

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Amortized cost excluding accrued interest, of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

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Amortized cost excluding accrued interest of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 310
-Name Accounting Standards Codification
-Section 45
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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
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-Name Accounting Standards Codification
-Section 50
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