Statutory Accounting Principles - Statutory Capital and Surplus for Principal Operating Subsidiaries (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Bermuda
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 1,150.6	$ 1,024.9
United Kingdom
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	0.0	370.0
United States (Federal)
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 1,410.4	$ 1,111.1

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479989/944-505-50-1

Name:	us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant