Derivative Instruments - Diesel Derivative Contracts Outstanding (Details) - Cash Flow Hedges gal in Millions	9 Months Ended
	Sep. 30, 2015 $ / gal gal
Diesel forward contracts
Derivative contracts
Diesel gallons (millions) | gal	31
Average rate ($/gallon)	2.43
Diesel forward contracts maturing in 2015
Derivative contracts
Diesel gallons (millions) | gal	6
Average rate ($/gallon)	2.53
Expected hedge ratio	66.00%
Diesel forward contracts maturing in 2016
Derivative contracts
Diesel gallons (millions) | gal	20
Average rate ($/gallon)	2.39
Expected hedge ratio	53.00%
Diesel forward contracts maturing in 2017
Derivative contracts
Diesel gallons (millions) | gal	5
Average rate ($/gallon)	2.47
Expected hedge ratio	16.00%