Financial Instruments and Risk Management (Derivative Instruments Change in Fair Value of Level 3) (Details) (Derivative Commodity Instruments, USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Derivative Commodity Instruments
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at beginning of period	$ 65,801,000		$ 42,755,000		$ 64,517,000
Realized gains (losses)	63,720,000		52,716,000		111,107,000
Unrealized gains relating to instruments held at the reporting date	22,160,000	[1]	23,980,000	[1]	(21,521,000)	[1]
Purchases and settlements during period	(62,662,000)		(53,650,000)		(111,348,000)
Balance at end of period	89,019,000		65,801,000		42,755,000
Unrealized gain (loss) on derivatives and commodity contracts	$ 19,900,000		$ 5,200,000

[1] Includes $19.9 million in mark-to-market gains and $5.2 million in mark-to-market losses for the years ended December 31, 2012 and 2011, respectively. There were no Level 3 mark-to-market gains or losses for the year ended December 31, 2010.