Loss and Loss Adjustment Expense Reserves - Cumulative Paid Loss and Allocated LAE, Net of Reinsurance (Details) - USD ($) $ in Millions	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	Dec. 31, 2012
Property and Accident & Health
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 337.1
All outstanding liabilities before 2012, net of reinsurance	0.6
Loss and LAE reserves, net of reinsurance	175.0
Property and Accident & Health | 2012
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	59.5	$ 59.4	$ 59.3	$ 58.7	$ 58.6	$ 58.2	$ 57.3	$ 53.6	$ 45.0	$ 14.2
Property and Accident & Health | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	61.2	61.3	61.3	61.3	61.1	61.1	60.0	40.3	15.8
Property and Accident & Health | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	28.3	28.4	28.2	28.1	28.0	27.6	25.4	13.9
Property and Accident & Health | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	15.3	15.1	14.8	14.9	13.7	12.4	7.0
Property and Accident & Health | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	17.5	17.2	17.1	16.8	13.4	8.6
Property and Accident & Health | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	30.2	33.3	32.1	26.3	17.0
Property and Accident & Health | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	40.7	40.7	32.8	15.8
Property and Accident & Health | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	19.1	17.1	6.9
Property and Accident & Health | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	34.5	11.4
Property and Accident & Health | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	30.8
Marine & Energy
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	161.7
All outstanding liabilities before 2012, net of reinsurance	1.2
Loss and LAE reserves, net of reinsurance	99.3
Marine & Energy | 2012
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	31.0	31.3	31.4	31.7	31.3	30.6	30.2	27.4	24.0	8.1
Marine & Energy | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	29.9	29.7	29.9	29.9	29.7	29.1	28.1	22.6	7.9
Marine & Energy | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	13.8	14.0	13.7	14.5	14.4	13.5	12.4	6.0
Marine & Energy | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	11.2	10.8	10.7	11.3	9.8	8.0	4.0
Marine & Energy | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	14.0	13.4	13.3	12.8	10.1	5.6
Marine & Energy | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	14.4	14.4	13.1	11.3	5.2
Marine & Energy | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	15.2	14.5	12.9	2.7
Marine & Energy | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	12.9	10.9	3.4
Marine & Energy | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	12.9	3.2
Marine & Energy | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	6.4
Specialty
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	145.3
All outstanding liabilities before 2012, net of reinsurance	(2.0)
Loss and LAE reserves, net of reinsurance	85.2
Specialty | 2012
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	24.5	24.4	24.4	24.2	23.3	23.2	22.6	22.2	25.2	16.3
Specialty | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	16.0	16.0	16.0	16.0	15.8	15.8	15.2	13.6	17.1
Specialty | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	43.5	43.6	42.8	41.5	40.8	40.4	39.7	26.8
Specialty | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	8.3	8.3	8.2	8.2	7.8	7.2	4.1
Specialty | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	10.5	10.4	10.0	9.2	8.0	3.2
Specialty | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	8.5	8.5	8.4	6.8	3.3
Specialty | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	10.3	9.8	8.1	2.9
Specialty | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	7.6	7.1	4.9
Specialty | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	10.9	5.4
Specialty | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	5.2
Casualty - Active
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	45.0
All outstanding liabilities before 2012, net of reinsurance	0.2
Loss and LAE reserves, net of reinsurance	37.4
Casualty - Active | 2012
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	9.1	8.9	9.1	8.5	8.0	7.4	6.4	4.8	3.3	1.3
Casualty - Active | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	7.2	7.0	7.0	6.7	6.3	5.7	5.3	3.6	1.5
Casualty - Active | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	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	5.1	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	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	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	3.5	1.4	0.3
Casualty - Active | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	2.3	1.4	0.3
Casualty - Active | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	1.0	0.5
Casualty - Active | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	0.5
Casualty - Runoff
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	216.9
All outstanding liabilities before 2012, net of reinsurance	21.4
Loss and LAE reserves, net of reinsurance	68.4
Casualty - Runoff | 2012
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	21.8	21.2	20.8	20.5	19.3	17.8	16.5	13.6	10.4	$ 3.5
Casualty - Runoff | 2013
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	46.0	45.6	45.0	44.3	42.7	41.0	35.9	19.5	$ 7.2
Casualty - Runoff | 2014
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	49.5	48.7	47.0	43.1	36.5	29.1	23.2	$ 6.5
Casualty - Runoff | 2015
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	28.2	26.5	23.9	20.3	14.0	7.9	$ 4.3
Casualty - Runoff | 2016
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	26.8	24.5	21.8	16.8	9.7	$ 3.8
Casualty - Runoff | 2017
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	21.1	18.3	14.2	9.1	$ 3.1
Casualty - Runoff | 2018
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	14.3	12.2	7.2	$ 3.3
Casualty - Runoff | 2019
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	7.4	5.6	$ 3.2
Casualty - Runoff | 2020
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	1.3	$ 0.8
Casualty - Runoff | 2021
Claims Development [Line Items]
Cumulative Paid Loss and LAE, Net of Reinsurance	$ 0.5