Fair Value Measurements (Summary of Assets and Liabilities Accounted at Fair Value on Recurring Basis) (Details) - Commodity Forward Contracts [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	$ 12,416	$ 14,986
Risk management liabilities - Commodity forward contracts	7,618	7,221
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	12,416	14,986
Risk management liabilities - Commodity forward contracts	7,618	7,221
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	233	878
Risk management liabilities - Commodity forward contracts	6,183	6,854
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	12,183	14,108
Risk management liabilities - Commodity forward contracts	$ 1,435	$ 367