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Restructuring - Additional Information (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2022
Sep. 30, 2023
Sep. 30, 2023
Restructuring Cost and Reserve [Line Items]      
Restructuring, description   The Company restructured its insurance operations to strengthen its market presence and enhance its focus on GBLI’s core Package Specialty and Targeted Specialty products. As a result, the Company exited its four brokerage divisions: Professional Liability, Excess Casualty, Environmental, and Middle Market Property. The Company ceased writing new business and existing renewals were placed in run-off for these four divisions. The restructuring plan, which was initiated in the fourth quarter of 2022, was completed in the first quarter of 2023.  
Restructuring, initiation date   Dec. 31, 2022  
Restructuring, completion date   Mar. 31, 2023  
Restructuring cost $ 3.4 $ 2.1 $ 5.5