Fair Value Measurements (Summary of Assets and Liabilities Accounted at Fair Value on Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 120,602	$ 0
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Hedged portion of firm commitment assets		399
Hedged portion of firm commitment liabilities	(3,208)	(13,032)
Available-for-sale securities	120,602
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Hedged portion of firm commitment assets		0
Hedged portion of firm commitment liabilities	0	0
Available-for-sale securities	14,998
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Hedged portion of firm commitment assets		399
Hedged portion of firm commitment liabilities	(3,208)	(13,032)
Available-for-sale securities	105,604
Recurring | Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk management assets - Commodity forward contracts	5,749	14,427
Risk management liabilities - Commodity forward contracts	(2,090)	(13,694)
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	1,808	1,395
Risk management liabilities - Commodity forward contracts	(74)	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	3,941	13,032
Risk management liabilities - Commodity forward contracts	$ (2,016)	$ (13,694)