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, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	$ 2,678.3
Total reinsurance recoverables on unpaid losses and LAE	2,434.9
Unallocated loss adjustment expenses	62.3
Unamortized reserve discount	(17.9)	$ (16.9)	$ (17.6)
Reserves for losses and loss adjustment expenses	5,157.6	4,654.6	4,201.0
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	913.0
Total reinsurance recoverables on unpaid losses and LAE	385.5
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	467.5
Total reinsurance recoverables on unpaid losses and LAE	332.2
Unamortized reserve discount	(13.0)	(11.9)	(10.6)
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	216.9
Total reinsurance recoverables on unpaid losses and LAE	175.1
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	52.7
Total reinsurance recoverables on unpaid losses and LAE	29.0
International Operations [Member] | Reinsurance Division [Member] | Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	85.7
Total reinsurance recoverables on unpaid losses and LAE	418.2
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	85.0
Total reinsurance recoverables on unpaid losses and LAE	175.4
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	93.3
Total reinsurance recoverables on unpaid losses and LAE	58.3
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	98.2
Total reinsurance recoverables on unpaid losses and LAE	79.9
International Operations [Member] | Syndicate 1200 [Member] | Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	117.9
Total reinsurance recoverables on unpaid losses and LAE	117.0
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	156.1
Total reinsurance recoverables on unpaid losses and LAE	112.0
International Operations [Member] | Europe [Member] | Professional [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	143.0
Total reinsurance recoverables on unpaid losses and LAE	7.8
Run Off Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	173.3
Total reinsurance recoverables on unpaid losses and LAE	85.1
Unamortized reserve discount	(4.9)	$ (5.0)	$ (7.0)
Other Lines [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total liabilities for unpaid losses and ALAE, net of reinsurance	75.7
Total reinsurance recoverables on unpaid losses and LAE	$ 459.4