Investments - Components of Restricted Assets (Detail) - Asset Pledged as Collateral - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	$ 192.4	$ 262.6
Securities Deposits for Regulatory and Other Purposes
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	152.7	153.4
Letter of Credit
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Restricted investments	$ 39.7	$ 109.2

The aggregate fair value as of the balance sheet date of financial instruments and other positions owned by the entity including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities. Includes both pledged and unpledged holdings.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481961/940-320-45-2

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant