Reserves for Losses and Loss Adjustment Expenses - Summary of Reconciliation of Net Incurred and Paid Development to Liability for Unpaid Losses and LAE in Consolidated Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	$ 3,071.6
Total reinsurance recoverables on unpaid losses and LAE	2,471.8
Unallocated loss adjustment expenses	70.4
Unamortized reserve discount	(18.8)	$ (17.8)	$ (17.9)
Reserves for losses and loss adjustment expenses	5,595.0	5,406.0	5,157.6
U.S. Operations | Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	1,540.7
Total reinsurance recoverables on unpaid losses and LAE	825.4
U.S. Operations | Professional
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	411.3
Total reinsurance recoverables on unpaid losses and LAE	251.3
U.S. Operations | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	68.8
Total reinsurance recoverables on unpaid losses and LAE	206.9
U.S. Operations | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	36.5
Total reinsurance recoverables on unpaid losses and LAE	11.4
International Operations | Argo Insurance Bermuda- Liability | Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	84.0
Total reinsurance recoverables on unpaid losses and LAE	207.6
International Operations | Syndicate 1200 | Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	56.2
Total reinsurance recoverables on unpaid losses and LAE	30.8
International Operations | Syndicate 1200 | Professional
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	78.6
Total reinsurance recoverables on unpaid losses and LAE	74.6
International Operations | Syndicate 1200 | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	139.1
Total reinsurance recoverables on unpaid losses and LAE	102.6
International Operations | Syndicate 1200 | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	170.4
Total reinsurance recoverables on unpaid losses and LAE	103.3
Run-off Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	183.4
Total reinsurance recoverables on unpaid losses and LAE	82.5
Unamortized reserve discount	(4.7)	$ (4.9)	$ (4.9)
Other Lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	302.6
Total reinsurance recoverables on unpaid losses and LAE	$ 575.4