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Employee Benefit Plans - Retired Officers' Bonus Plan and Retired VP Bonus Plan (Details) - USD ($)
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2015
Other Postretirement Benefits Plan        
Defined Benefit Plan Disclosure [Line Items]        
Benefits paid $ 3,419,000 $ 3,333,000 $ 3,020,000  
Benefit obligation 126,886,000 118,089,000 114,008,000 $ 107,317,000
Fair value of plan assets 0 0 0 $ 0
Other Postretirement Benefits Plan | Officer        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan, funded (unfunded) status of plan 3,800,000 4,400,000    
Retired Officers' Bonus Plan | Other Postretirement Benefits Plan | Officer        
Defined Benefit Plan Disclosure [Line Items]        
Retired Officers' Bonus Plan, total pension expense 700,000 700,000 700,000  
Benefits paid 1,500,000 900,000 $ 300,000  
Retired Vice Presidents' Bonus Plan        
Defined Benefit Plan Disclosure [Line Items]        
Benefit obligation 900,000 1,000,000    
Retired Vice Presidents' Bonus Plan | Other Postretirement Benefits Plan | Officer        
Defined Benefit Plan Disclosure [Line Items]        
Retired Officers' Bonus Plan, total pension expense 200,000 0    
Benefits paid $ 0 $ 0