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, 2024 USD ($) claim	Dec. 31, 2023 USD ($)	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 ($)	Dec. 31, 2015 USD ($)
Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	$ 1,207.6
Cumulative Paid Loss and LAE, Net of Reinsurance	672.4
All outstanding liabilities before 2015, net of reinsurance	1.5
Loss and LAE reserves, net of reinsurance	536.7
Marine & Energy
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	765.7	$ 307.3
All outstanding liabilities before 2015, net of reinsurance	4.7
Loss and LAE reserves, net of reinsurance	463.1
Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	716.0
Cumulative Paid Loss and LAE, Net of Reinsurance	299.6
All outstanding liabilities before 2015, net of reinsurance	0.6
Loss and LAE reserves, net of reinsurance	417.0
Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	237.6
Cumulative Paid Loss and LAE, Net of Reinsurance	64.5
All outstanding liabilities before 2015, net of reinsurance	4.9
Loss and LAE reserves, net of reinsurance	178.0
Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	177.9
Cumulative Paid Loss and LAE, Net of Reinsurance	139.6
All outstanding liabilities before 2015, net of reinsurance	28.2
Loss and LAE reserves, net of reinsurance	66.5
2015 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	16.0	$ 15.7	$ 15.8	$ 16.0	$ 16.0	$ 16.2	$ 17.2	$ 18.1	$ 19.1
Total IBNR plus expected development on reported claims	$ 0.1
Cumulative number of reported claims | claim	2,829
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.6	15.4	15.0	15.0	14.8	14.5	14.6	13.4	12.2	6.9
2015 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	12.7	12.7	12.9	12.7	12.8	12.7	13.3	16.1	17.4	21.7
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | claim	3,243
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 11.6	11.6	11.5	10.9	10.5	10.4	11.0	9.6	7.8	4.0
2015 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	11.3	8.1	8.9	11.1	11.2	11.0	10.7	12.3	14.6	17.3
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims | claim	1,841
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.3	6.2	6.4	8.1	8.1	8.0	8.0	7.6	7.0	4.0
2015 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	7.4	7.5	7.3	7.0	7.1	7.4	8.1	8.2	9.7	9.6
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	1,306
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 6.3	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	39.4	40.2	39.1	36.7	37.3	36.3	36.6	33.1	31.9	36.4
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims | claim	1,950
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 35.8	35.3	33.0	28.9	27.3	24.6	21.4	14.5	8.2	$ 4.3
2016 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	18.5	18.5	18.5	18.5	18.3	18.4	18.2	17.5	22.2
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	3,433
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.1	17.9	17.7	17.1	16.8	16.7	16.3	13.0	8.5
2016 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.3	13.6	14.1	14.8	14.3	14.6	15.6	19.5	23.4
Total IBNR plus expected development on reported claims	$ 0.0
Cumulative number of reported claims | claim	3,772
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 13.4	13.4	13.4	13.7	13.1	13.0	12.6	10.0	5.5
2016 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	12.3	8.5	8.9	11.8	11.8	11.3	10.9	14.3	18.2
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	1,936
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 11.7	8.3	8.5	10.3	10.3	9.9	9.1	7.9	3.2
2016 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	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	$ 0.5
Cumulative number of reported claims | claim	1,588
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 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	37.4	37.3	37.6	38.4	38.7	38.4	40.3	32.1	32.4
Total IBNR plus expected development on reported claims	$ 2.0
Cumulative number of reported claims | claim	2,150
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 32.4	30.9	28.7	27.8	25.3	22.7	17.7	10.1	$ 3.9
2017 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	43.4	43.8	42.3	42.8	44.2	45.2	37.7	31.1
Total IBNR plus expected development on reported claims	$ 15.8
Cumulative number of reported claims | claim	4,624
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 28.2	25.4	27.1	29.4	32.7	31.5	25.7	16.8
2017 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.1	16.0	15.8	16.6	16.9	17.6	19.3	26.0
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	4,139
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 14.3	14.0	14.1	14.1	14.0	12.8	11.1	5.1
2017 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.9	10.3	10.6	11.6	11.4	11.9	12.8	17.9
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	2,195
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.6	8.9	9.2	8.5	8.5	8.3	6.5	3.1
2017 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	10.9	10.6	10.5	9.0	9.3	10.8	11.7	11.5
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims | claim	1,667
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 8.3	7.5	5.7	4.2	3.4	2.7	1.7	0.8
2017 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	28.4	28.2	29.8	31.5	32.0	31.3	33.8	30.5
Total IBNR plus expected development on reported claims	$ 2.3
Cumulative number of reported claims | claim	1,604
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 23.5	22.8	22.5	21.3	18.4	14.6	9.4	$ 3.2
2018 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	46.4	46.3	46.8	46.7	49.0	47.1	40.7
Total IBNR plus expected development on reported claims	$ 1.9
Cumulative number of reported claims | claim	4,288
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 43.6	42.8	40.8	40.0	40.1	32.2	15.6
2018 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.7	17.7	17.3	17.8	17.4	19.9	25.4
Total IBNR plus expected development on reported claims	$ 0.2
Cumulative number of reported claims | claim	3,238
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.4	15.3	15.3	14.6	13.9	12.4	2.6
2018 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.6	15.7	14.8	15.9	16.6	16.2	14.4
Total IBNR plus expected development on reported claims	$ 0.4
Cumulative number of reported claims | claim	2,122
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 14.1	13.0	11.8	10.4	9.9	8.2	2.7
2018 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.4	9.1	8.6	10.8	11.1	13.3	12.9
Total IBNR plus expected development on reported claims	$ 1.1
Cumulative number of reported claims | claim	1,147
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 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	27.7	27.9	26.1	26.5	27.2	28.1	33.5
Total IBNR plus expected development on reported claims	$ 3.4
Cumulative number of reported claims | claim	1,280
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 21.3	18.2	16.2	14.9	12.6	7.4	$ 3.4
2019 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	22.8	23.2	23.8	27.0	31.2	33.9
Total IBNR plus expected development on reported claims	$ 0.6
Cumulative number of reported claims | claim	4,024
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 20.6	19.3	18.5	18.3	16.7	6.8
2019 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	21.2	21.9	21.4	21.6	21.6	23.7
Total IBNR plus expected development on reported claims	$ 0.5
Cumulative number of reported claims | claim	2,393
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 18.1	15.3	14.3	12.6	10.6	3.3
2019 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	19.9	30.1	25.6	18.6	19.4	21.6
Total IBNR plus expected development on reported claims	$ 0.3
Cumulative number of reported claims | claim	2,387
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 17.6	25.1	18.2	7.4	6.9	4.8
2019 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	13.0	11.4	10.6	12.3	13.7	14.8
Total IBNR plus expected development on reported claims	$ 1.7
Cumulative number of reported claims | claim	1,019
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 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	23.6	23.5	24.8	23.3	23.2	26.4
Total IBNR plus expected development on reported claims	$ 5.2
Cumulative number of reported claims | claim	973
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 15.8	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	79.5	77.6	74.5	75.1	76.9
Total IBNR plus expected development on reported claims	$ 9.9
Cumulative number of reported claims | claim	4,646
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 66.5	55.6	46.9	33.9	11.2
2020 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	27.1	27.4	28.5	27.1	29.7
Total IBNR plus expected development on reported claims	$ 0.8
Cumulative number of reported claims | claim	1,582
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 21.9	18.5	16.0	12.7	3.1
2020 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.7	19.5	18.6	22.8	23.7
Total IBNR plus expected development on reported claims	$ 0.9
Cumulative number of reported claims | claim	2,017
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 17.7	17.7	12.9	10.5	5.0
2020 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	8.8	9.2	10.8	12.0	13.5
Total IBNR plus expected development on reported claims	$ 2.0
Cumulative number of reported claims | claim	665
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 5.3	3.3	2.0	1.0	0.5
2020 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	9.5	10.9	13.8	12.2	15.8
Total IBNR plus expected development on reported claims	$ 2.9
Cumulative number of reported claims | claim	567
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 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	168.0	165.3	153.7	170.0
Total IBNR plus expected development on reported claims	$ 7.5
Cumulative number of reported claims | claim	3,509
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 142.7	129.8	86.5	30.7
2021 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	74.5	67.3	69.3	86.2
Total IBNR plus expected development on reported claims	$ 4.0
Cumulative number of reported claims | claim	1,505
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 51.9	38.2	24.3	6.3
2021 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	43.8	51.4	62.1	70.3
Total IBNR plus expected development on reported claims	$ 10.0
Cumulative number of reported claims | claim	1,725
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 34.6	35.5	23.9	5.0
2021 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	16.3	16.6	22.4	21.4
Total IBNR plus expected development on reported claims	$ 6.4
Cumulative number of reported claims | claim	961
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 9.6	3.1	0.9	0.5
2021 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	4.5	5.4	7.0	10.4
Total IBNR plus expected development on reported claims	$ 1.8
Cumulative number of reported claims | claim	283
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 2.3	1.8	1.7	$ 0.5
2022 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	277.3	266.9	241.5
Total IBNR plus expected development on reported claims	$ 12.9
Cumulative number of reported claims | claim	4,044
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 229.4	191.9	69.4
2022 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	156.0	153.6	149.7
Total IBNR plus expected development on reported claims	$ 31.9
Cumulative number of reported claims | claim	1,968
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 97.7	66.2	12.2
2022 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	168.4	174.8	180.3
Total IBNR plus expected development on reported claims	$ 77.0
Cumulative number of reported claims | claim	1,496
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 82.5	61.7	16.0
2022 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	34.6	38.0	32.9
Total IBNR plus expected development on reported claims	$ 27.9
Cumulative number of reported claims | claim	1,558
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 2.4	1.5	0.4
2022 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	2.5	2.6	0.8
Total IBNR plus expected development on reported claims	$ 1.6
Cumulative number of reported claims | claim	80
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.7	0.5	$ 0.3
2023 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	176.1	213.9
Total IBNR plus expected development on reported claims	$ 77.3
Cumulative number of reported claims | claim	3,598
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 52.9	19.9
2023 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	188.2	197.0
Total IBNR plus expected development on reported claims	$ 115.9
Cumulative number of reported claims | claim	2,138
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 42.1	10.5
2023 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	197.4	214.5
Total IBNR plus expected development on reported claims	$ 94.4
Cumulative number of reported claims | claim	1,641
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 75.2	18.4
2023 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	65.9	60.9
Total IBNR plus expected development on reported claims	$ 57.4
Cumulative number of reported claims | claim	1,792
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 5.6	0.9
2023 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	3.6	2.7
Total IBNR plus expected development on reported claims	$ 2.4
Cumulative number of reported claims | claim	40
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.0	$ 0.9
2024 | Property and Accident & Health
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	359.5
Total IBNR plus expected development on reported claims	$ 202.7
Cumulative number of reported claims | claim	3,614
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 54.8
2024 | Marine & Energy
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	239.9
Total IBNR plus expected development on reported claims	$ 182.6
Cumulative number of reported claims | claim	1,485
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 20.9
2024 | Specialty
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	218.7
Total IBNR plus expected development on reported claims	$ 141.6
Cumulative number of reported claims | claim	1,292
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 27.3
2024 | Casualty-Active
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	59.5
Total IBNR plus expected development on reported claims	$ 54.9
Cumulative number of reported claims | claim	1,170
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 1.9
2024 | Casualty-Runoff
Claims Development [Line Items]
Incurred Loss and LAE, Net of Reinsurance	1.3
Total IBNR plus expected development on reported claims	$ 0.7
Cumulative number of reported claims | claim	22
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.5