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Quarterly Financial Data (Unaudited) - Quarterly Charges and Items Affecting Comparability (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
Jun. 30, 2016
Dec. 31, 2017
Quarterly Data              
Net earnings related to tax reform legislation             $ 37,423,000
Gas-fired power plant projects | U.S.              
Quarterly Data              
Effect of forecast revision on estimated project cost $ 41,000,000   $ 194,000,000 $ 25,000,000      
Effect of forecast revision on estimated project cost, in dollars per diluted share $ 0.19   $ 0.89 $ 0.11      
Downstream project | U.S.              
Quarterly Data              
Effect of forecast revision on estimated project cost $ 29,000,000 $ 9,000,000 $ 6,000,000        
Effect of forecast revision on estimated project cost, in dollars per diluted share $ 0.13 $ 0.04 $ 0.03        
Net earnings related to tax reform legislation $ 37,000,000            
Net earnings related to tax reform legislation, in dollars per diluted share $ 0.27            
Petrochemicals project | U.S.              
Quarterly Data              
Effect of forecast revision on estimated project cost         $ 241,000,000 $ 24,000,000  
Effect of forecast revision on estimated project cost, in dollars per diluted share         $ 1.10 $ 0.10