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Variable Interest Entities (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Assets, Current $ 3,074,392       $ 2,837,756       $ 3,074,392 $ 2,837,756  
Liabilities, Current 1,581,846       1,518,943       1,581,846 1,518,943  
REVENUE 1,193,068 $ 1,199,505 $ 1,247,274 $ 1,117,361 $ 1,246,599 $ 1,332,978 $ 1,308,130 $ 1,085,369 $ 4,757,208 $ 4,973,076 $ 4,920,472
O&G [Member]                      
Ownership percentage in joint venture                 75.00%    
Related party's ownership percentage in joint venture                 25.00%    
Purple Line Segment 2 Expansion Project [Member] | Scenario, Plan [Member]                      
REVENUE                 $ 1,400,000    
Purple Line Segment 2 Expansion Project [Member] | O&G [Member]                      
Ownership percentage in joint venture                 75.00%    
Related party's ownership percentage in joint venture                 25.00%    
Variable Interest Entity, Primary Beneficiary [Member]                      
Assets, Current 95,500               $ 95,500    
Assets, Noncurrent 11,600               11,600    
Liabilities, Current $ 140,700               $ 140,700