Statutory Accounting Principles - Statutory Net Income (Loss) for Principal Operating Subsidiaries (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Bermuda
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 14.2	$ (18.0)	$ 7.1
United Kingdom
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	8.6	(53.0)	(15.1)
United States (Federal)
Statutory Accounting Practices [Line Items]
Statutory net income (loss)	$ 103.5	$ 76.4	$ 196.1

Amount of net income for the period determined using accounting principles prescribed or permitted by insurance regulators.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration