Financial Derivative Instruments - Fuel Hedging (Details) gal in Millions	Sep. 30, 2018 gal	[1]
Remainder of Current Year
Volume of Fuel Hedging
Fuel Hedged (in gallons)	412
2019
Volume of Fuel Hedging
Fuel Hedged (in gallons)	1,377
2020
Volume of Fuel Hedging
Fuel Hedged (in gallons)	867
2021
Volume of Fuel Hedging
Fuel Hedged (in gallons)	466
2022
Volume of Fuel Hedging
Fuel Hedged (in gallons)	88

[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.