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Employee Benefit Plans (Components of Net Postretirement Medical Expense) (Details) - Officer Medical Plan - USD ($)
$ in Thousands
12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 4,851 $ 5,702 $ 4,086
Interest cost 4,782 4,505 3,568
Net actuarial loss 3,052 3,536 582
Total postretirement medical expense $ 12,685 $ 13,743 $ 8,236