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Regulatory Requirements (Detail)
$ in Thousands
9 Months Ended
Sep. 30, 2021
USD ($)
Regulatory Assets [Line Items]  
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 $ 669,400
Amount of capital in excess of aggregate regulatory requirements 360,500
Disposal Group, Held-for-sale, Not Discontinued Operations  
Regulatory Assets [Line Items]  
Net assets held by regulated subsidiaries 47,100
Amount of capital in excess of aggregate regulatory requirements $ 38,600