Fair Value Measurements (Summary of Assets and Liabilities Accounted at Fair Value on Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for Sale	$ 0	$ 239,388
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for Sale	21,066	239,388
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for Sale	21,066	91,595
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for Sale	0	147,793
Recurring | Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	451	296
Risk management liabilities - Commodity forward contracts	(6,477)	(176)
Recurring | Forward Contracts [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	428	48
Risk management liabilities - Commodity forward contracts	(119)	0
Recurring | Forward Contracts [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	23	248
Risk management liabilities - Commodity forward contracts	$ (6,358)	$ (176)