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, 2021
U.S. Operations | Liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	8.90%
Year 2	17.60%
Year 3	18.20%
Year 4	18.30%
Year 5	11.50%
Year 6	8.30%
Year 7	5.20%
Year 8	3.60%
Year 9	2.40%
Year 10	1.70%
Remainder	4.30%
U.S. Operations | Professional
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	6.10%
Year 2	19.10%
Year 3	22.00%
Year 4	18.60%
Year 5	12.50%
Year 6	8.90%
Year 7	5.50%
Year 8	3.20%
Year 9	1.80%
Year 10	1.00%
Remainder	1.20%
U.S. Operations | Property
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	58.40%
Year 2	32.50%
Year 3	5.70%
Year 4	2.30%
Year 5	0.70%
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	52.70%
Year 2	22.80%
Year 3	19.20%
Year 4	3.40%
Year 5	1.20%
Year 6	0.40%
Year 7	0.10%
Year 8	0.10%
Year 9	0.00%
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	0.60%
Year 2	14.20%
Year 3	12.70%
Year 4	13.20%
Year 5	12.20%
Year 6	10.50%
Year 7	8.60%
Year 8	6.80%
Year 9	5.30%
Year 10	4.00%
Remainder	11.90%
International Operations | Syndicate 1200 | Liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	6.30%
Year 2	11.50%
Year 3	13.40%
Year 4	15.00%
Year 5	16.00%
Year 6	11.70%
Year 7	6.90%
Year 8	5.20%
Year 9	3.70%
Year 10	2.70%
Remainder	7.60%
International Operations | Syndicate 1200 | Professional
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	7.30%
Year 2	12.50%
Year 3	18.50%
Year 4	17.60%
Year 5	12.40%
Year 6	8.70%
Year 7	5.90%
Year 8	4.50%
Year 9	3.20%
Year 10	2.30%
Remainder	7.10%
International Operations | Syndicate 1200 | Property
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	33.80%
Year 2	39.20%
Year 3	14.30%
Year 4	8.40%
Year 5	2.70%
Year 6	0.80%
Year 7	0.40%
Year 8	0.20%
Year 9	0.10%
Year 10	0.00%
Remainder	0.10%
International Operations | Syndicate 1200 | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	32.00%
Year 2	36.40%
Year 3	16.70%
Year 4	9.20%
Year 5	3.00%
Year 6	1.50%
Year 7	0.60%
Year 8	0.30%
Year 9	0.10%
Year 10	0.00%
Remainder	0.00%