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Employee Thrift and Defined Benefit Plan - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Employee Thrift Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Bank's expense related to the thrift plan $ 848,000 $ 633,000 $ 545,000
Thrift Plan owns outstanding shares 319,994    
Percentage of outstanding shares owns with thrift plan 2.70%    
Pentegra Defined Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Bank's expense related to the thrift plan $ 0    
Pentegra Plan date Aug. 01, 2007    
Employer identification number 135645888    
Plan number 333    
Withdrawal liability $ 4,500,000