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Loss and Loss Adjustment Expense Reserves - Incurred Loss and Allocated LAE, Net of Reinsurance (Details) - Ark $ in Millions	Dec. 31, 2023 USD ($) claim	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	$ 908.0
Cumulative Paid Loss and LAE, Net of Reinsurance	551.2
All outstanding liabilities before 2014, net of reinsurance	1.9
Loss and LAE reserves, net of reinsurance	358.7
Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	580.4
Cumulative Paid Loss and LAE, Net of Reinsurance	240.7
All outstanding liabilities before 2014, net of reinsurance	0.0
Loss and LAE reserves, net of reinsurance	339.7
Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	543.9
Cumulative Paid Loss and LAE, Net of Reinsurance	218.5
All outstanding liabilities before 2014, net of reinsurance	6.3
Loss and LAE reserves, net of reinsurance	331.7
Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	181.5
Cumulative Paid Loss and LAE, Net of Reinsurance	49.2
All outstanding liabilities before 2014, net of reinsurance	4.8
Loss and LAE reserves, net of reinsurance	137.1
Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	239.1
Cumulative Paid Loss and LAE, Net of Reinsurance	186.7
All outstanding liabilities before 2014, net of reinsurance	28.9
Loss and LAE reserves, net of reinsurance	81.3
2014 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.5	$ 28.6	$ 28.6	$ 28.6	$ 28.6	$ 28.5	$ 28.7	$ 29.4	$ 29.5	$ 32.8
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims | claim	2,924
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 28.0	28.0	28.0	28.0	27.9	27.7	27.6	27.3	25.1	13.7
2014 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	43.5	43.5	43.7	43.8	43.8	41.2	40.8	41.2	44.2	46.0
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | claim	1,359
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 43.7	43.4	43.1	43.2	42.4	41.1	40.5	40.1	39.3	26.5
2014 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.6	13.9	13.8	14.0	13.7	14.1	16.3	17.2	20.2	34.4
Total IBNR plus expected development on reported claims	$ (0.3)
Cumulative number of reported claims | claim	2,572
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.8	13.9	13.6	13.8	13.6	14.3	14.2	13.4	12.3	5.9
2014 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	7.7	7.8	7.6	7.8	7.7	8.1	8.2	8.4	9.4	13.3
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims | claim	1,393
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.7	6.2	6.0	5.9	5.5	5.2	4.7	4.2	3.5	1.3
2014 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	59.7	58.7	58.9	59.1	59.1	57.6	54.0	51.0	48.4	49.2
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims | claim	1,947
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 54.9	51.9	49.4	48.7	47.0	43.2	36.5	29.6	23.2	$ 6.5
2015 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	15.8	15.9	16.2	16.1	16.3	17.3	18.3	19.3
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	2,829
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.5	15.1	15.1	14.9	14.7	14.7	13.5	12.3	6.9
2015 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	8.2	9.0	11.3	11.3	11.1	10.8	12.4	14.8	17.4
Total IBNR plus expected development on reported claims	$ (0.1)
Cumulative number of reported claims | claim	1,841
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.3	6.5	8.2	8.2	8.1	8.1	7.7	7.1	4.0
2015 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	12.9	13.1	12.8	13.0	12.8	13.4	16.3	17.5	21.8
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims | claim	3,242
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 11.7	11.6	11.0	10.6	10.5	11.1	9.7	7.9	4.0
2015 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	7.5	7.3	7.0	7.2	7.4	8.1	8.2	9.8	9.6
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	1,295
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.1	5.5	5.1	4.9	4.7	4.4	3.2	2.4	1.8
2015 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	40.3	39.3	36.8	37.5	36.5	36.8	33.3	32.1	36.7
Total IBNR plus expected development on reported claims	$ 2.0
Cumulative number of reported claims | claim	2,000
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 35.3	33.1	29.0	27.4	24.7	21.4	14.5	8.2	$ 4.3
2016 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	18.7	18.8	18.8	18.6	18.6	18.5	17.7	22.5
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	3,428
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.1	17.9	17.3	17.0	16.9	16.5	13.2	8.6
2016 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	8.6	9.0	11.9	11.9	11.3	11.0	14.4	18.4
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims | claim	1,932
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.4	8.6	10.4	10.4	9.9	9.1	8.0	3.2
2016 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.8	14.2	15.0	14.4	14.7	15.8	19.7	23.6
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims | claim	3,770
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.6	13.6	13.9	13.3	13.1	12.7	10.1	5.6
2016 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.1	9.2	9.2	9.1	9.0	8.9	8.3	8.8
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	1,553
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.1	6.5	5.3	4.6	4.0	2.3	1.0	0.2
2016 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	37.5	37.7	38.6	38.8	38.6	40.4	32.2	32.6
Total IBNR plus expected development on reported claims	$ 2.3
Cumulative number of reported claims | claim	2,158
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 31.0	28.7	27.8	25.4	22.7	17.7	10.2	$ 3.9
2017 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	44.2	42.7	43.2	44.6	45.6	38.1	31.3
Total IBNR plus expected development on reported claims	$ 15.3
Cumulative number of reported claims | claim	4,613
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 25.7	27.5	29.8	33.0	31.8	26.0	16.9
2017 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.4	10.7	11.7	11.6	12.1	13.0	18.1
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims | claim	2,191
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.1	9.3	8.6	8.6	8.5	6.7	3.2
2017 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	15.9	16.8	17.1	17.7	19.5	26.2
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims | claim	4,133
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 14.1	14.2	14.2	14.2	12.9	11.2	5.1
2017 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.7	10.5	9.1	9.4	10.8	11.7	11.6
Total IBNR plus expected development on reported claims	$ 1.1
Cumulative number of reported claims | claim	1,632
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 7.5	5.7	4.2	3.4	2.8	1.7	0.8
2017 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.3	29.9	31.6	32.1	31.4	34.0	30.6
Total IBNR plus expected development on reported claims	$ 2.9
Cumulative number of reported claims | claim	1,603
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 22.8	22.5	21.4	18.5	14.7	9.4	$ 3.2
2018 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	46.7	47.1	47.0	49.3	47.4	41.0
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims | claim	4,269
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 43.1	41.1	40.3	40.3	32.4	15.7
2018 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	15.8	14.9	16.1	16.8	16.4	14.6
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	2,117
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.1	11.9	10.5	10.1	8.3	2.8
2018 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	18.1	17.7	18.1	17.8	20.3	25.8
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	3,225
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.6	15.6	15.0	14.3	12.7	2.7
2018 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.2	8.6	10.9	11.1	13.3	12.9
Total IBNR plus expected development on reported claims	$ 1.4
Cumulative number of reported claims | claim	1,098
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.2	4.3	4.3	3.5	1.4	0.3
2018 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.0	26.2	26.6	27.3	28.2	33.7
Total IBNR plus expected development on reported claims	$ 4.1
Cumulative number of reported claims | claim	1,277
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.2	16.3	14.9	12.6	7.4	$ 3.4
2019 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	23.4	24.0	27.2	31.4	34.2
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims | claim	4,011
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 19.4	18.6	18.5	16.8	6.8
2019 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	30.1	25.7	18.7	19.6	21.8
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	2,368
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 25.2	18.3	7.5	7.0	4.9
2019 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	22.1	21.6	21.8	21.8	23.9
Total IBNR plus expected development on reported claims	$ 0.9
Cumulative number of reported claims | claim	2,366
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.5	14.5	12.7	10.7	3.4
2019 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	11.4	10.6	12.4	13.7	14.8
Total IBNR plus expected development on reported claims	$ 2.6
Cumulative number of reported claims | claim	954
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 5.7	3.0	2.3	1.4	0.3
2019 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	23.6	24.9	23.4	23.3	26.6
Total IBNR plus expected development on reported claims	$ 5.8
Cumulative number of reported claims | claim	971
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.2	12.2	7.8	5.8	$ 3.3
2020 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	78.4	75.1	75.8	77.6
Total IBNR plus expected development on reported claims	$ 12.4
Cumulative number of reported claims | claim	4,617
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 56.1	47.3	34.3	11.3
2020 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	19.9	19.0	23.2	24.2
Total IBNR plus expected development on reported claims	$ 1.7
Cumulative number of reported claims | claim	1,994
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.3	13.2	10.7	5.3
2020 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	27.6	28.7	27.3	30.0
Total IBNR plus expected development on reported claims	$ 1.6
Cumulative number of reported claims | claim	1,561
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.7	16.2	12.8	3.2
2020 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.2	10.9	12.1	13.5
Total IBNR plus expected development on reported claims	$ 4.3
Cumulative number of reported claims | claim	605
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 3.3	2.0	1.0	0.5
2020 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.9	13.9	12.3	15.9
Total IBNR plus expected development on reported claims	$ 3.9
Cumulative number of reported claims | claim	566
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.0	3.1	1.3	$ 0.8
2021 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	167.3	155.5	171.8
Total IBNR plus expected development on reported claims	$ 5.7
Cumulative number of reported claims | claim	3,465
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 131.3	87.1	30.9
2021 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	51.7	62.6	71.2
Total IBNR plus expected development on reported claims	$ 16.4
Cumulative number of reported claims | claim	1,696
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 35.7	24.1	5.1
2021 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	67.7	69.8	86.8
Total IBNR plus expected development on reported claims	$ 9.3
Cumulative number of reported claims | claim	1,461
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 38.5	24.5	6.3
2021 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.6	22.4	21.4
Total IBNR plus expected development on reported claims	$ 12.4
Cumulative number of reported claims | claim	842
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 3.1	0.9	0.5
2021 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	5.5	7.0	10.5
Total IBNR plus expected development on reported claims	$ 3.0
Cumulative number of reported claims | claim	282
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.9	1.7	$ 0.5
2022 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	270.5	244.3
Total IBNR plus expected development on reported claims	$ 14.3
Cumulative number of reported claims | claim	3,899
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 193.9	70.9
2022 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	176.6	181.1
Total IBNR plus expected development on reported claims	$ 97.2
Cumulative number of reported claims | claim	1,407
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 62.1	16.0
2022 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	154.2	150.2
Total IBNR plus expected development on reported claims	$ 50.4
Cumulative number of reported claims | claim	1,802
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 66.4	12.3
2022 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	38.1	33.0
Total IBNR plus expected development on reported claims	$ 34.4
Cumulative number of reported claims | claim	1,237
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.6	0.4
2022 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	2.6	0.8
Total IBNR plus expected development on reported claims	$ 1.8
Cumulative number of reported claims | claim	77
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.5	$ 0.3
2023 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	214.2
Total IBNR plus expected development on reported claims	$ 156.9
Cumulative number of reported claims | claim	2,963
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 20.1
2023 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	215.6
Total IBNR plus expected development on reported claims	$ 151.6
Cumulative number of reported claims | claim	1,258
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.8
2023 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	197.8
Total IBNR plus expected development on reported claims	$ 154.8
Cumulative number of reported claims | claim	1,539
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 10.6
2023 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	61.0
Total IBNR plus expected development on reported claims	$ 57.9
Cumulative number of reported claims | claim	1,065
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.9
2023 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	2.7
Total IBNR plus expected development on reported claims	$ 1.6
Cumulative number of reported claims | claim	40
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.9

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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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- Definition

Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (b)
-SubTopic 40
-Topic 944
-Publisher FASB
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- Definition

Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4D
-Subparagraph (a)
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- Definition

Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (a)
-SubTopic 40
-Topic 944
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-URI https://asc.fasb.org//1943274/2147480081/944-40-50-4B

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- Definition

Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-SubTopic 40
-Topic 944
-Publisher FASB
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- Definition

Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense.

+ References

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-Publisher FASB
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