Long-term Debt- Schedule of Terms and Principal Amounts of Other Indebtedness (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Debt Instrument [Line Items]
Other indebtedness	$ 0.0	$ 57.0
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Argo Underwriting Agency Limited
Debt Instrument [Line Items]
Other indebtedness	(54.7)	0.0
Floating rate loan stock
Debt Instrument [Line Items]
Floating rate loan stock	$ 54.7	$ 57.0

Amount classified as other liabilities attributable to disposal group held for sale or disposed of.

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