Loss and Loss Adjustment Expense Reserves - Incurred Loss and Allocated LAE, Net of Reinsurance (Details) - P&C Insurance and Reinsurance (Ark/WM Outrigger) $ in Millions	Dec. 31, 2025 USD ($) claim	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	$ 1,518.1
Cumulative Paid Loss and LAE, Net of Reinsurance	942.2
All outstanding liabilities before 2016, net of reinsurance	2.4
Loss and LAE reserves, net of reinsurance	578.3
Marine & Energy
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	1,001.6	$ 439.2
All outstanding liabilities before 2016, net of reinsurance	6.6
Loss and LAE reserves, net of reinsurance	569.0
Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	918.1
Cumulative Paid Loss and LAE, Net of Reinsurance	411.0
All outstanding liabilities before 2016, net of reinsurance	1.1
Loss and LAE reserves, net of reinsurance	508.2
Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	303.2
Cumulative Paid Loss and LAE, Net of Reinsurance	76.2
All outstanding liabilities before 2016, net of reinsurance	3.6
Loss and LAE reserves, net of reinsurance	230.6
Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	137.0
Cumulative Paid Loss and LAE, Net of Reinsurance	110.9
All outstanding liabilities before 2016, net of reinsurance	31.0
Loss and LAE reserves, net of reinsurance	57.1
2016 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	18.8	18.8	$ 18.8	$ 18.8	$ 18.9	$ 18.7	$ 18.7	$ 18.6	$ 17.8	$ 22.7
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	3,434
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.4	18.4	18.2	18.0	17.4	17.1	17.0	16.6	13.3	8.6
2016 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.8	13.7	14.0	14.5	15.2	14.6	14.9	16.0	20.0	23.8
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | claim	3,776
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.8	13.8	13.8	13.8	14.1	13.4	13.3	12.9	10.2	5.6
2016 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	12.2	12.5	8.6	9.1	12.0	12.0	11.4	11.1	14.5	18.5
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	1,936
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 11.8	11.8	8.4	8.7	10.5	10.4	10.0	9.2	8.0	3.2
2016 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	11.9	11.8	10.1	9.2	9.2	9.1	9.0	8.9	8.3	8.8
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	1,606
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 10.3	9.7	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	38.1	37.5	37.5	37.7	38.6	38.9	38.6	40.5	32.3	32.6
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims | claim	2,156
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 34.1	32.5	31.0	28.8	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	43.3	43.9	44.3	42.9	43.4	44.8	45.8	38.3	31.6
Total IBNR plus expected development on reported claims	$ 15.6
Cumulative number of reported claims | claim	4,629
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 29.3	28.7	25.9	27.7	29.9	33.1	31.9	26.1	17.0
2017 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.4	16.4	16.3	16.0	16.9	17.3	17.9	19.7	26.5
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	4,141
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.9	14.6	14.3	14.3	14.4	14.4	13.1	11.3	5.2
2017 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	11.1	11.2	10.6	10.9	11.9	11.8	12.3	13.2	18.3
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | claim	2,201
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.9	9.9	9.3	9.5	8.8	8.8	8.6	6.8	3.3
2017 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.6	10.9	10.7	10.5	9.1	9.3	10.8	11.7	11.6
Total IBNR plus expected development on reported claims	$ 0.7
Cumulative number of reported claims | claim	1,693
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.1	8.3	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	27.9	28.5	28.3	29.9	31.6	32.1	31.5	34.0	30.7
Total IBNR plus expected development on reported claims	$ 2.0
Cumulative number of reported claims | claim	1,606
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 24.8	23.5	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	49.0	47.0	47.0	47.4	47.4	49.7	47.8	41.4
Total IBNR plus expected development on reported claims	$ 2.7
Cumulative number of reported claims | claim	4,293
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 45.2	44.2	43.4	41.4	40.6	40.6	32.7	15.8
2018 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	17.0	17.4	18.4	18.0	18.5	18.1	20.6	26.2
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	3,245
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 16.3	16.1	15.9	16.0	15.3	14.6	12.9	2.7
2018 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	17.5	16.9	15.9	15.1	16.2	17.0	16.6	14.7
Total IBNR plus expected development on reported claims	$ 0.9
Cumulative number of reported claims | claim	2,129
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 14.8	14.4	13.2	12.0	10.6	10.2	8.5	2.9
2018 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.5	9.4	9.2	8.6	10.9	11.1	13.3	12.9
Total IBNR plus expected development on reported claims	$ 0.9
Cumulative number of reported claims | claim	1,175
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.0	7.1	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	28.2	27.8	28.0	26.2	26.6	27.3	28.2	33.6
Total IBNR plus expected development on reported claims	$ 2.9
Cumulative number of reported claims | claim	1,284
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 22.7	21.3	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	21.5	23.2	23.5	24.2	27.4	31.7	34.6
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	4,030
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 19.9	20.9	19.6	18.8	18.6	17.0	6.9
2019 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	21.8	21.6	22.3	21.9	22.1	22.0	24.2
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	2,413
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 19.3	18.4	15.7	14.7	12.9	10.9	3.4
2019 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	26.8	20.0	30.1	25.9	18.9	19.8	22.1
Total IBNR plus expected development on reported claims	$ 0.7
Cumulative number of reported claims | claim	2,396
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 19.8	17.7	25.2	18.5	7.6	7.2	5.0
2019 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	14.4	13.0	11.4	10.6	12.3	13.7	14.8
Total IBNR plus expected development on reported claims	$ 1.4
Cumulative number of reported claims | claim	1,062
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 10.2	8.3	5.7	3.0	2.3	1.4	0.3
2019 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	22.5	23.7	23.6	24.9	23.4	23.3	26.5
Total IBNR plus expected development on reported claims	$ 4.2
Cumulative number of reported claims | claim	976
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 17.3	15.9	15.1	12.1	7.8	5.8	$ 3.3
2020 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	84.8	81.0	79.1	75.3	76.3	78.1
Total IBNR plus expected development on reported claims	$ 6.0
Cumulative number of reported claims | claim	4,664
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 72.4	67.4	56.3	47.5	34.5	11.4
2020 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	27.6	27.6	27.9	29.1	27.7	30.4
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims | claim	1,594
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 23.2	22.4	18.9	16.3	12.9	3.2
2020 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	17.7	17.3	20.4	19.3	23.5	24.5
Total IBNR plus expected development on reported claims	$ 1.3
Cumulative number of reported claims | claim	2,028
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 19.2	18.5	18.8	13.4	11.0	5.4
2020 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.5	8.8	9.2	10.9	12.1	13.5
Total IBNR plus expected development on reported claims	$ 1.7
Cumulative number of reported claims | claim	731
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.0	5.3	3.3	2.0	1.0	0.5
2020 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.2	9.5	10.9	13.9	12.3	15.9
Total IBNR plus expected development on reported claims	$ 2.3
Cumulative number of reported claims | claim	573
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.5	6.3	6.0	3.1	1.3	$ 0.8
2021 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	169.7	171.5	168.9	156.8	173.0
Total IBNR plus expected development on reported claims	$ 8.1
Cumulative number of reported claims | claim	3,533
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 150.9	145.6	132.3	87.5	31.0
2021 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	79.8	75.6	68.4	70.5	87.7
Total IBNR plus expected development on reported claims	$ 3.8
Cumulative number of reported claims | claim	1,530
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 64.4	52.7	38.9	24.9	6.4
2021 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	41.7	44.0	51.8	63.0	71.8
Total IBNR plus expected development on reported claims	$ 6.6
Cumulative number of reported claims | claim	1,735
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 35.3	34.6	35.8	24.1	5.1
2021 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	21.7	16.3	16.6	22.4	21.4
Total IBNR plus expected development on reported claims	$ 4.1
Cumulative number of reported claims | claim	1,050
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 12.6	9.6	3.1	0.9	0.5
2021 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	3.9	4.5	5.5	7.0	10.5
Total IBNR plus expected development on reported claims	$ 1.5
Cumulative number of reported claims | claim	284
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 2.4	2.3	1.9	1.7	$ 0.5
2022 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	281.9	280.0	269.6	245.7
Total IBNR plus expected development on reported claims	$ 9.0
Cumulative number of reported claims | claim	4,100
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 250.2	232.0	194.5	71.7
2022 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	218.6	157.5	154.9	150.8
Total IBNR plus expected development on reported claims	$ 56.8
Cumulative number of reported claims | claim	2,083
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 145.5	98.6	66.7	12.3
2022 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	179.6	172.6	178.5	181.9
Total IBNR plus expected development on reported claims	$ 58.3
Cumulative number of reported claims | claim	1,577
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 113.2	84.9	62.5	16.1
2022 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	17.3	34.7	38.1	33.0
Total IBNR plus expected development on reported claims	$ 9.8
Cumulative number of reported claims | claim	1,774
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 3.6	2.5	1.6	0.5
2022 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	2.3	2.6	2.6	0.8
Total IBNR plus expected development on reported claims	$ 1.3
Cumulative number of reported claims | claim	80
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.9	0.7	0.6	$ 0.3
2023 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	130.4	178.6	216.9
Total IBNR plus expected development on reported claims	$ 20.8
Cumulative number of reported claims | claim	3,688
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 78.5	53.8	20.2
2023 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	123.2	189.9	198.6
Total IBNR plus expected development on reported claims	$ 36.8
Cumulative number of reported claims | claim	2,341
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 61.3	42.7	10.6
2023 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	153.4	199.4	216.6
Total IBNR plus expected development on reported claims	$ 41.3
Cumulative number of reported claims | claim	1,734
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 109.1	76.3	19.2
2023 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	57.1	65.9	61.0
Total IBNR plus expected development on reported claims	$ 39.4
Cumulative number of reported claims | claim	2,234
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 10.8	5.6	0.9
2023 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	3.1	3.6	2.7
Total IBNR plus expected development on reported claims	$ 1.8
Cumulative number of reported claims | claim	40
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.1	1.0	$ 0.9
2024 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	324.6	364.8
Total IBNR plus expected development on reported claims	$ 106.4
Cumulative number of reported claims | claim	4,495
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 134.6	55.4
2024 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	248.1	243.3
Total IBNR plus expected development on reported claims	$ 138.4
Cumulative number of reported claims | claim	2,195
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 60.1	21.4
2024 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	231.3	223.2
Total IBNR plus expected development on reported claims	$ 133.2
Cumulative number of reported claims | claim	1,798
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 53.8	28.9
2024 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	82.0	60.0
Total IBNR plus expected development on reported claims	$ 71.9
Cumulative number of reported claims | claim	2,171
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 4.6	1.9
2024 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	1.1	1.2
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	20
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.5	$ 0.5
2025 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	394.1
Total IBNR plus expected development on reported claims	$ 171.7
Cumulative number of reported claims | claim	3,899
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 142.8
2025 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	235.3
Total IBNR plus expected development on reported claims	$ 184.9
Cumulative number of reported claims | claim	1,618
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 19.4
2025 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	226.8
Total IBNR plus expected development on reported claims	$ 169.4
Cumulative number of reported claims | claim	1,831
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 24.1
2025 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	68.2
Total IBNR plus expected development on reported claims	$ 63.4
Cumulative number of reported claims | claim	1,723
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.0
2025 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	0.7
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | claim	41
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.6