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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
 6 Months Ended 9 Months Ended
Jan. 01, 2020
Jun. 30, 2020
Sep. 30, 2020
Dec. 31, 2019
Summary Of Significant Accounting Policies [Line Items]        
Net decrease in retained earnings     $ 735,218,000 $ 682,692,000
Transition adjustment, allowance for credit loses for loans $ 1,800,000   1,800,000  
Adjustment of transition reserve for loans     $ 41,000  
Increase in provision for credit loses   $ 23,500,000    
Revision of Prior Period, Accounting Standards Update, Adjustment [Member]        
Summary Of Significant Accounting Policies [Line Items]        
Net decrease in retained earnings $ 1,300,000