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Employee Thrift and Defined Benefit Plan - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Employer identification number 35-1562417    
Employee Thrift Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Bank's expense related to the thrift plan $ 1,200,000 $ 1,200,000 $ 942,000
Thrift Plan owns outstanding shares 741,650    
Percentage of outstanding shares owns with thrift plan 1.60%    
Pentegra Defined Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Bank's expense related to the thrift plan $ 0 $ 0  
Pentegra Plan date Aug. 01, 2007    
Employer identification number 13-5645888    
Plan number 333    
Withdrawal liability $ 3,100,000