Financial Instruments and Risk Management (Derivative Instruments Change in Fair Value of Level 3) (Details) (Derivative Commodity Instruments, USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Dec. 31, 2010	Dec. 31, 2011	Dec. 31, 2009
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period		$ 42,755	$ 154,094
Realized (gains) losses	(241)	(6,180)	13
Unrealized gains relating to instruments held at the reporting date	90,580	79,882	65,015
Purchases and settlements during period	(112,101)	(50,656)	(154,605)
Balance at end of period	$ 42,755	$ 65,801	$ 64,517