Financial Instruments - Outstanding Fuel Hedges (Details)	Sep. 30, 2015 gal $ / gal
Fuel Hedges Remaining 2015 [Member]
Summary of costless collar hedges outstanding
Gallons hedged (in gallons per year)	6,750,000
Weighted Average Contract Price per Gallon (in dollars per gallon) | $ / gal	3.76
Fuel Hedges Remaining 2016 [Member]
Summary of costless collar hedges outstanding
Gallons hedged (in gallons per year)	27,000,000
Weighted Average Contract Price per Gallon (in dollars per gallon) | $ / gal	3.57
Fuel Hedges Remaining 2017 [Member]
Summary of costless collar hedges outstanding
Gallons hedged (in gallons per year)	12,000,000
Weighted Average Contract Price per Gallon (in dollars per gallon) | $ / gal	2.92