Fair Value Measurements - Narrative (Details)	12 Months Ended
	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Significant valuation assumptions	Volatilities used in these valuations ranged from 26% to 57% depending on the maturity dates, underlying commodities and strike prices of the option contracts.
Derivative [Member] | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assumed volatilities in valuation of fuel contracts	26.00%
Derivative [Member] | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assumed volatilities in valuation of fuel contracts	57.00%