Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2025	Dec. 31, 2024
Vintage year, Direct financing leases
2025	$ 41.0
2024	9.9	$ 48.3
2023	1.9	1.8
2022	120.2	126.3
2021	45.6	43.7
2020		90.0
Prior (prior period)		252.9
Prior (current period)	326.0
Direct financing leases, Total amortized cost	544.6	563.0
Reinsurance recoverable and deposit receivable	19,260.9	19,493.4
Accrued interest receivable on direct financing leases	1.5	1.2
Mortgage and other loans
Accrued interest receivable	482.0	647.3
A- and above
Vintage year, Direct financing leases
2024		1.0
2022	30.9	38.5
2021	11.0	11.0
2020		33.8
Prior (prior period)		177.4
Prior (current period)	203.7
Direct financing leases, Total amortized cost	245.6	261.7
BBB+ thru BBB-
Vintage year, Direct financing leases
2024	9.9	4.9
2023		1.8
2022	88.8	87.3
2021	20.5	19.0
2020		52.8
Prior (prior period)		67.0
Prior (current period)	92.0
Direct financing leases, Total amortized cost	211.2	232.8
BB+ thru BB-
Vintage year, Direct financing leases
2024		38.8
2023	1.9
2022	0.5	0.5
2021	5.3	6.8
2020		3.4
Prior (prior period)		8.5
Prior (current period)	30.3
Direct financing leases, Total amortized cost	38.0	58.0
B+ and below
Vintage year, Direct financing leases
2025	41.0
2024		3.6
2021	8.8	6.9
Direct financing leases, Total amortized cost	49.8	10.5
Commercial mortgage loans
Vintage year, Mortgage loans
2025	1,304.4
2024	1,606.3	1,607.8
2023	1,286.9	1,330.1
2022	1,466.0	1,620.2
2021	2,017.6	2,418.3
2020		1,686.6
Prior (current period)	8,967.4
Prior (prior period)		8,409.7
Mortgage loans, Total amortized cost	16,648.6	17,072.7
Mortgage and other loans
Accrued interest receivable	75.1	69.2
Commercial mortgage loans \| A- and above
Vintage year, Mortgage loans
2025	1,054.0
2024	1,308.8	1,182.1
2023	936.7	793.1
2022	1,095.7	1,234.3
2021	1,794.1	2,101.3
2020		1,463.4
Prior (current period)	7,124.3
Prior (prior period)		6,594.0
Mortgage loans, Total amortized cost	13,313.6	13,368.2
Commercial mortgage loans \| BBB+ thru BBB-
Vintage year, Mortgage loans
2025	164.1
2024	164.0	210.0
2023	206.1	393.6
2022	155.7	231.6
2021	175.4	269.1
2020		180.6
Prior (current period)	1,202.3
Prior (prior period)		1,217.5
Mortgage loans, Total amortized cost	2,067.6	2,502.4
Commercial mortgage loans \| BB+ thru BB-
Vintage year, Mortgage loans
2025	86.3
2024	133.5	215.7
2023	109.5	143.4
2022	214.6	154.3
2021	48.1	47.9
2020		40.1
Prior (current period)	243.9
Prior (prior period)		271.7
Mortgage loans, Total amortized cost	835.9	873.1
Commercial mortgage loans \| B+ and below
Vintage year, Mortgage loans
2023	34.6
2020		2.5
Prior (current period)	396.9
Prior (prior period)		326.5
Mortgage loans, Total amortized cost	431.5	329.0
Residential mortgage loans
Vintage year, Mortgage loans
2025 mortgage loans excluding portfolio layer method basis adjustments	787.5
2024 mortgage loans excluding portfolio layer method basis adjustments	398.6	350.9
2023 mortgage loans excluding portfolio layer method basis adjustments	399.4	417.0
2022 mortgage loans excluding portfolio layer method basis adjustments	939.4	976.4
2021 mortgage loans excluding portfolio layer method basis adjustments	1,158.1	1,239.7
2020 mortgage loans excluding portfolio layer method basis adjustments		198.5
Prior mortgage loans excluding portfolio layer method basis adjustments (prior period)		433.3
Prior mortgage loans excluding portfolio layer method basis adjustments (current period)	573.5
Total mortgage loans excluding portfolio layer method basis adjustments	4,256.5	3,615.8
Unallocated portfolio layer method basis adjustment	(0.9)	(8.4)
Mortgage loans, Total amortized cost	4,255.6	3,607.4
Mortgage and other loans
Accrued interest receivable	17.7	12.0
Residential mortgage loans \| Performing
Vintage year, Mortgage loans
2025	787.5
2024	397.3	350.4
2023	394.2	411.5
2022	933.2	970.3
2021	1,151.7	1,234.9
2020		196.0
Prior (current period)	570.1
Prior (prior period)		427.6
Mortgage loans, Total amortized cost	4,234.0	3,590.7
Residential mortgage loans \| Non-performing
Vintage year, Mortgage loans
2024	1.3	0.5
2023	5.2	5.5
2022	6.2	6.1
2021	6.4	4.8
2020		2.5
Prior (current period)	3.4
Prior (prior period)		5.7
Mortgage loans, Total amortized cost	$ 22.5	25.1
Mortgage and other loans
Mortgage loans, delinquent to be considered non-performing (in days)	90 days
Other loans
Vintage year, Mortgage loans
2025	$ 79.6
2024	37.9	84.3
2023	36.0	61.9
Prior (prior period)		0.1
Mortgage loans, Total amortized cost	153.5	146.3
Mortgage and other loans
Accrued interest receivable	1.2	1.3
Other loans \| Performing
Vintage year, Mortgage loans
2025	79.5
2024	34.8	84.2
2023	34.5	61.9
Mortgage loans, Total amortized cost	148.8	146.1
Other loans \| Non-performing
Vintage year, Mortgage loans
2025	0.1
2024	3.1	0.1
2023	1.5
Prior (prior period)		0.1
Mortgage loans, Total amortized cost	$ 4.7	$ 0.2

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

+ References

No definition available.

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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.

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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.

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

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No definition available.

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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.

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

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

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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

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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.

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

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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.

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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.

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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.

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No definition available.

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

+ References

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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
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-Name Accounting Standards Codification
-Section 50
-Paragraph 6
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-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

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

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

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

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

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

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

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

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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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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.

+ References

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