Fair Value Measurements (Summary of Assets and Liabilities Accounted at Fair Value on Recurring Basis) (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 5,074	$ 15,414
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	5,074	15,414
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0
Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	4,872	3,145
Risk management liabilities - Commodity forward contracts	(6,493)	(10,108)
Forward Contracts [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	3,958	3,143
Risk management liabilities - Commodity forward contracts	(1,825)	0
Forward Contracts [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	914	2
Risk management liabilities - Commodity forward contracts	$ (4,668)	$ (10,108)