Reserves for Losses and Loss Adjustment Expenses - Schedule of Supplementary Unaudited Information About Annual Percentage Payout of Incurred Losses and ALAE, Net of Reinsurance (Detail)	Dec. 31, 2022
U.S. Operations | Liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	7.80%
Year 2	17.00%
Year 3	18.50%
Year 4	19.40%
Year 5	12.40%
Year 6	8.60%
Year 7	6.10%
Year 8	3.00%
Year 9	2.10%
Year 10	1.50%
Remainder	3.60%
U.S. Operations | Professional
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	5.40%
Year 2	17.00%
Year 3	19.40%
Year 4	17.50%
Year 5	12.20%
Year 6	9.80%
Year 7	6.70%
Year 8	4.40%
Year 9	2.80%
Year 10	1.80%
Remainder	2.80%
U.S. Operations | Property
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	56.60%
Year 2	33.10%
Year 3	6.70%
Year 4	2.50%
Year 5	0.80%
Year 6	0.20%
Year 7	0.10%
Year 8	0.00%
Year 9	0.00%
Year 10	0.00%
Remainder	0.00%
U.S. Operations | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	27.90%
Year 2	32.40%
Year 3	23.50%
Year 4	10.00%
Year 5	4.00%
Year 6	1.50%
Year 7	0.50%
Year 8	0.20%
Year 9	0.10%
Year 10	0.00%
Remainder	0.00%
International Operations | Argo Insurance Bermuda- Liability | Liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	0.50%
Year 2	14.70%
Year 3	19.60%
Year 4	15.20%
Year 5	10.70%
Year 6	7.70%
Year 7	5.60%
Year 8	4.30%
Year 9	3.40%
Year 10	2.70%
Remainder	15.60%
International Operations | Argo Insurance Bermuda- Liability | Professional
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	2.70%
Year 2	7.60%
Year 3	11.80%
Year 4	13.70%
Year 5	13.50%
Year 6	11.90%
Year 7	9.80%
Year 8	7.70%
Year 9	5.90%
Year 10	4.40%
Remainder	11.10%