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Major Customers and Suppliers
 12 Months Ended
Dec. 31, 2017
Major Customers and Suppliers  
Major Customers and Suppliers
(20) Major Customers and Suppliers

Sales to major customers as a percentage of the respective segment's sales were as follows:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Petroleum
 
 
 
 
 
Customer A
19
%
 
15
%
 
14
%
Nitrogen Fertilizer
 
 
 
 
 
Customer B
5
%
 
10
%
 
10
%
Customer C
11
%
 
10
%
 
14
%
 
16
%
 
20
%
 
24
%


The petroleum segment obtained crude oil from one third-party supplier under a long-term supply agreement during 2017, 2016 and 2015. Volume contracted as a percentage of the total crude oil purchases (in barrels) for each of the periods was as follows:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Petroleum
 
 
 
 
 
Supplier A
55
%
 
61
%
 
61
%