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Fair Value Measurements (Narrative) (Detail) - $ / gal
Sep. 30, 2018
Dec. 31, 2017
Fair Value [Abstract]    
Minimum range of DOE index curve used in DCF model 3.36 2.95
Maximum range of DOE index curve used in DCF model 3.39 3.00
Weighted average DOE index curve used in DCF model 3.38 2.96