Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Dec. 31, 2024
Vintage year, Direct financing leases
2025	$ 40.5
2024	9.9	$ 48.3
2023	2.0	1.8
2022	132.7	126.3
2021	45.9	43.7
2020		90.0
Prior (prior period)		252.9
Prior (current period)	356.9
Direct financing leases, Total amortized cost	587.9	563.0
Reinsurance recoverable and deposit receivable	19,395.8	19,493.4
Accrued interest receivable on direct financing leases	1.5	1.2
Mortgage and other loans
Accrued interest receivable	475.6	647.3
A- and above
Vintage year, Direct financing leases
2024		1.0
2022	42.3	38.5
2021	11.1	11.0
2020		33.8
Prior (prior period)		177.4
Prior (current period)	232.5
Direct financing leases, Total amortized cost	285.9	261.7
BBB+ thru BBB-
Vintage year, Direct financing leases
2024	9.9	4.9
2023		1.8
2022	89.8	87.3
2021	20.5	19.0
2020		52.8
Prior (prior period)		67.0
Prior (current period)	93.7
Direct financing leases, Total amortized cost	213.9	232.8
BB+ thru BB-
Vintage year, Direct financing leases
2025	40.5
2024		38.8
2023	2.0
2022	0.6	0.5
2021	7.0	6.8
2020		3.4
Prior (prior period)		8.5
Prior (current period)	30.7
Direct financing leases, Total amortized cost	80.8	58.0
B+ and below
Vintage year, Mortgage loans
2024		3.6
2021		6.9
Mortgage loans, Total amortized cost		10.5
Vintage year, Direct financing leases
2021	7.3
Direct financing leases, Total amortized cost	7.3
Commercial mortgage loans
Vintage year, Mortgage loans
2025	134.8
2024	1,488.7	1,607.8
2023	1,392.2	1,330.1
2022	1,507.1	1,620.2
2021	2,396.7	2,418.3
2020		1,686.6
Prior (current period)	9,739.9
Prior (prior period)		8,409.7
Mortgage loans, Total amortized cost	16,659.4	17,072.7
Mortgage and other loans
Accrued interest receivable	70.8	69.2
Commercial mortgage loans \| A- and above
Vintage year, Mortgage loans
2025	82.0
2024	1,188.9	1,182.1
2023	795.0	793.1
2022	1,100.7	1,234.3
2021	2,077.4	2,101.3
2020		1,463.4
Prior (current period)	7,779.5
Prior (prior period)		6,594.0
Mortgage loans, Total amortized cost	13,023.5	13,368.2
Commercial mortgage loans \| BBB+ thru BBB-
Vintage year, Mortgage loans
2025	13.0
2024	211.5	210.0
2023	450.8	393.6
2022	249.7	231.6
2021	270.9	269.1
2020		180.6
Prior (current period)	1,393.4
Prior (prior period)		1,217.5
Mortgage loans, Total amortized cost	2,589.3	2,502.4
Commercial mortgage loans \| BB+ thru BB-
Vintage year, Mortgage loans
2025	39.8
2024	88.3	215.7
2023	146.4	143.4
2022	156.7	154.3
2021	48.4	47.9
2020		40.1
Prior (current period)	237.6
Prior (prior period)		271.7
Mortgage loans, Total amortized cost	717.2	873.1
Commercial mortgage loans \| B+ and below
Vintage year, Mortgage loans
2020		2.5
Prior (current period)	329.4
Prior (prior period)		326.5
Mortgage loans, Total amortized cost	329.4	329.0
Residential mortgage loans
Vintage year, Mortgage loans
2025 mortgage loans excluding portfolio layer method basis adjustments	90.5
2024 mortgage loans excluding portfolio layer method basis adjustments	477.1	350.9
2023 mortgage loans excluding portfolio layer method basis adjustments	436.0	417.0
2022 mortgage loans excluding portfolio layer method basis adjustments	961.3	976.4
2021 mortgage loans excluding portfolio layer method basis adjustments	1,215.2	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)	619.5
Total mortgage loans excluding portfolio layer method basis adjustments	3,799.6	3,615.8
Unallocated portfolio layer method basis adjustment	(3.1)	(8.4)
Mortgage loans, Total amortized cost	3,796.5	3,607.4
Mortgage and other loans
Accrued interest receivable	12.9	12.0
Residential mortgage loans \| Performing
Vintage year, Mortgage loans
2025	90.5
2024	477.1	350.4
2023	432.2	411.5
2022	954.8	970.3
2021	1,209.8	1,234.9
2020		196.0
Prior (current period)	615.0
Prior (prior period)		427.6
Mortgage loans, Total amortized cost	3,779.4	3,590.7
Residential mortgage loans \| Non-performing
Vintage year, Mortgage loans
2024		0.5
2023	3.8	5.5
2022	6.5	6.1
2021	5.4	4.8
2020		2.5
Prior (current period)	4.5
Prior (prior period)		5.7
Mortgage loans, Total amortized cost	$ 20.2	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	$ 14.9
2024	63.9	84.3
2023	52.3	61.9
Prior (current period)	0.1
Prior (prior period)		0.1
Mortgage loans, Total amortized cost	131.2	146.3
Mortgage and other loans
Accrued interest receivable	1.1	1.3
Other loans \| Performing
Vintage year, Mortgage loans
2025	14.4
2024	63.9	84.2
2023	52.3	61.9
Mortgage loans, Total amortized cost	130.6	146.1
Other loans \| Non-performing
Vintage year, Mortgage loans
2025	0.5
2024		0.1
Prior (current period)	0.1
Prior (prior period)		0.1
Mortgage loans, Total amortized cost	$ 0.6	$ 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.

+ 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.

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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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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.

+ References

No definition available.

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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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No definition available.

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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

No definition available.

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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.

+ References

No definition available.

+ Details

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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.

+ 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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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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- 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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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
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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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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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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