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Employee Benefit Plans (Components of net periodic post retirement benefit cost) (Details) - USD ($)
$ in Thousands
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Postemployment Benefits [Abstract]      
Service cost for benefits earned during the period $ 1,073 $ 1,026 $ 961
Interest cost on accumulated postretirement benefit 869 814 752
Other components 58 88 35
Net periodic postretirement benefit cost $ 2,000 $ 1,928 $ 1,748