Long-term Debt - Schedule of Junior Subordinated Debentures (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 243.2	$ 241.2
Argo Group
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	$ 27.2	$ 14.3
Trust preferred debentures | Argo Group | PXRE Capital Statutory Trust II
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
Interest rate	8.89%	9.74%
Junior subordinated debentures, Amount	$ 18.0	$ 18.0
Trust preferred debentures | Argo Group | PXRE Capital Trust VI
Debt Instrument [Line Items]
Interest rate basis spread	3.90%	3.90%
Interest rate	8.49%	9.49%
Junior subordinated debentures, Amount	$ 10.3	$ 10.3
Trust preferred debentures | Argo Group U.S.
Debt Instrument [Line Items]
Junior subordinated debentures, Amount	162.0	160.8
Less: fair value adjustment	$ (10.7)	$ (11.9)
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust I
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
Interest rate	8.89%	9.74%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust III
Debt Instrument [Line Items]
Interest rate basis spread	4.10%	4.10%
Interest rate	9.02%	9.76%
Junior subordinated debentures, Amount	$ 12.3	$ 12.3
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust IV
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
Interest rate	8.64%	9.49%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust V
Debt Instrument [Line Items]
Interest rate basis spread	3.85%	3.85%
Interest rate	8.63%	9.49%
Junior subordinated debentures, Amount	$ 12.4	$ 12.4
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust VI
Debt Instrument [Line Items]
Interest rate basis spread	3.80%	3.80%
Interest rate	8.41%	9.47%
Junior subordinated debentures, Amount	$ 13.4	$ 13.4
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust VII
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
Interest rate	8.22%	9.25%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust VIII
Debt Instrument [Line Items]
Interest rate basis spread	3.55%	3.55%
Interest rate	8.15%	9.18%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust IX
Debt Instrument [Line Items]
Interest rate basis spread	3.60%	3.60%
Interest rate	8.22%	9.25%
Junior subordinated debentures, Amount	$ 15.5	$ 15.5
Trust preferred debentures | Argo Group U.S. | Argonaut Group Statutory Trust X
Debt Instrument [Line Items]
Interest rate basis spread	3.40%	3.40%
Interest rate	8.02%	9.05%
Junior subordinated debentures, Amount	$ 30.9	$ 30.9