Loss and Loss Adjustment Expense Reserves - Cumulative Paid Loss and Allocated LAE, Net of Reinsurance (Details) - Ark - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property and Accident & Health
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 410.2
All outstanding liabilities before 2013, net of reinsurance	1.4
Loss and LAE reserves, net of reinsurance	258.2	$ 175.0
Property and Accident & Health | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	59.2	59.2	$ 59.3	$ 59.3	$ 59.4	$ 59.1	$ 59.1	$ 58.1	$ 39.1	$ 15.4
Property and Accident & Health | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	27.9	27.8	27.9	27.8	27.6	27.5	27.1	24.9	13.6
Property and Accident & Health | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	15.0	15.0	14.8	14.6	14.6	13.4	12.2	6.9
Property and Accident & Health | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	17.8	17.2	16.9	16.8	16.4	13.1	8.5
Property and Accident & Health | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	27.3	29.6	32.8	31.6	25.8	16.8
Property and Accident & Health | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	40.8	40.0	40.1	32.2	15.6
Property and Accident & Health | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	18.5	18.3	16.7	6.8
Property and Accident & Health | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	47.0	34.1	11.2
Property and Accident & Health | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	86.7	30.8
Property and Accident & Health | 2022
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	70.0
Marine & Energy
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	164.0
All outstanding liabilities before 2013, net of reinsurance	4.0
Loss and LAE reserves, net of reinsurance	196.4	99.3
Marine & Energy | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	29.3	29.3	29.1	29.3	29.3	29.1	28.6	27.6	22.2	7.8
Marine & Energy | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	13.7	13.5	13.6	13.4	14.1	14.0	13.2	12.1	5.8
Marine & Energy | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	11.4	10.8	10.4	10.3	10.9	9.6	7.8	4.0
Marine & Energy | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	13.4	13.7	13.1	13.0	12.6	10.0	5.5
Marine & Energy | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	14.1	14.1	14.0	12.8	11.1	5.1
Marine & Energy | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	15.4	14.7	14.0	12.5	2.7
Marine & Energy | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	14.3	12.6	10.6	3.3
Marine & Energy | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	16.0	12.7	3.1
Marine & Energy | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	24.2	6.3
Marine & Energy | 2022
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	12.2
Specialty
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	165.8
All outstanding liabilities before 2013, net of reinsurance	0.2
Loss and LAE reserves, net of reinsurance	204.3	85.2
Specialty | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	15.6	15.6	15.7	15.7	15.7	15.5	15.4	14.9	13.2	17.0
Specialty | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	43.0	42.7	42.8	42.0	40.7	40.1	39.7	38.9	26.3
Specialty | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	6.4	8.1	8.1	8.1	8.0	7.6	7.0	4.0
Specialty | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	8.5	10.3	10.3	9.9	9.1	7.9	3.2
Specialty | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	9.2	8.5	8.5	8.4	6.6	3.1
Specialty | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	11.8	10.4	10.0	8.2	2.7
Specialty | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	18.2	7.4	6.9	4.8
Specialty | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	13.0	10.6	5.2
Specialty | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	24.1	5.1
Specialty | 2022
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	16.0
Casualty - Active
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	42.0
All outstanding liabilities before 2013, net of reinsurance	4.0
Loss and LAE reserves, net of reinsurance	71.5	37.4
Casualty - Active | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	7.5	7.3	7.0	7.0	6.7	6.3	5.8	5.3	3.6	1.5
Casualty - Active | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	6.2	6.0	5.9	5.5	5.2	4.7	4.2	3.5	1.3
Casualty - Active | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	5.5	5.1	4.9	4.7	4.4	3.2	2.4	1.8
Casualty - Active | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	6.5	5.3	4.6	4.0	2.3	1.0	0.2
Casualty - Active | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	5.7	4.2	3.4	2.8	1.7	0.8
Casualty - Active | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	4.3	4.3	3.5	1.4	0.3
Casualty - Active | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	3.0	2.3	1.4	0.3
Casualty - Active | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	2.0	1.0	0.5
Casualty - Active | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	0.9	0.5
Casualty - Active | 2022
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	0.4
Casualty - Runoff
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	214.8
All outstanding liabilities before 2013, net of reinsurance	19.4
Loss and LAE reserves, net of reinsurance	60.8	68.4
Casualty - Runoff | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	45.2	44.9	44.6	43.9	43.3	42.4	40.6	35.7	19.4	$ 7.1
Casualty - Runoff | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	51.8	49.3	48.5	46.9	43.1	36.4	29.5	23.1	$ 6.4
Casualty - Runoff | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	33.1	28.9	27.3	24.7	21.4	14.5	8.2	$ 4.3
Casualty - Runoff | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	28.7	27.8	25.4	22.7	17.7	10.2	$ 3.9
Casualty - Runoff | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	22.5	21.4	18.5	14.6	9.4	$ 3.2
Casualty - Runoff | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	16.3	14.9	12.6	7.4	$ 3.4
Casualty - Runoff | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	12.1	7.8	5.8	$ 3.3
Casualty - Runoff | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	3.1	1.3	$ 0.8
Casualty - Runoff | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	1.7	$ 0.5
Casualty - Runoff | 2022
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.3