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Postretirement Medical Plan (Tables)
3 Months Ended
Mar. 31, 2015
Postretirement Medical Plan  
Schedule of components of net periodic postretirement benefit cost
Three Months Ended
March 31,
20152014
Service cost$ 1,229 $ 1,038
Interest cost 482 464
Amortization of prior service cost 313 247
Net periodic benefit cost$ 2,024 $ 1,749