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Financial Derivative Instruments - Fuel Hedging (Details)
gal in Millions
Dec. 31, 2017
gal
 [1]
2018  
Volume of Fuel Hedging [Line Items]  
Fuel Hedged (in gallons) 1,647
2019  
Volume of Fuel Hedging [Line Items]  
Fuel Hedged (in gallons) 1,377
2020  
Volume of Fuel Hedging [Line Items]  
Fuel Hedged (in gallons) 685
Beyond 2020  
Volume of Fuel Hedging [Line Items]  
Fuel Hedged (in gallons) 315
[1] Due to the types of derivatives utilized by the Company and different price levels of those contracts, these volumes represent the maximum economic hedge in place and may vary significantly as market prices fluctuate.