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Regulatory Requirements (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2022
USD ($)
Broker-Dealer, Net Capital Requirement, SEC Regulation [Abstract]  
Minimum period required for financial resources to cover operating costs 1 year
Minimum period required for cash or highly liquid securities to cover operating costs 6 months
Net assets held by regulated subsidiaries $ 637.7
Amount of capital in excess of aggregate regulatory requirements $ 306.0