Investments - Components of Restricted Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities pledged as collateral for letters of credit and other	$ 193.9	$ 189.4
Securities and cash on deposit supporting Lloyd’s business (1)	296.8	409.2
Total restricted investments	686.3	826.1
Securities And Letters Of Credit Pledged As Part Of Syndicate, Minimum Amount	296.8
Argo Re
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities and cash on deposit supporting Lloyd’s business (1)	140.3
Securities on deposit for regulatory and other purposes
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities on deposit for regulatory and other purposes	$ 195.6	$ 227.5

Securities and letters of credit pledged.

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- Definition

Securities And Letters Of Credit Pledged As Part Of Syndicate, Minimum Amount

+ References

No definition available.

+ Details

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The sum of the fair values, as of the date of each statement of financial position presented, of securities that are restricted as collateral for the payment of related debt obligations, primarily secured borrowings and repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

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- Definition

The fair value, as of the date of each statement of financial position presented, of securities that are restricted as collateral under letter of credit agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

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