Fair Value Measurements (Narrative) (Details) - Diesel swap gal in Millions	6 Months Ended
Jun. 30, 2017 $ / gal gal
Derivatives, Fair Value [Line Items]
Fixed price swap contract (in gallons) \| gal	4.4
Average contract price (in dollars per gallon)	2.57
Average forward price (in dollars per gallon)	2.51

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