Derivative Financial Instruments (Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Not Designated as Hedging Instrument [Member] | Fair Value [Member]
Derivative Instruments [Line Items]
Oil Barrels In Dollars	$ 479	$ 1,068
Oil Barrels (2)In Dollars	(384)	(963)
Natural gas MMBTUs In Dollars		3,540
Natural gas MMBTUs (2) In Dollars		(3,480)
Not Designated as Hedging Instrument [Member] | Notional Amount [Member]
Derivative Instruments [Line Items]
Oil Barrels	499,000	459,000
Oil Barrels (2)	499,000	459,000
Natural gas MMBTUs		1,905,000
Natural gas MMBTUs (2)		1,905,000
Non-Hedging Commodity Options [Member] | Fair Value [Member]
Derivative Instruments [Line Items]
Oil Barrels In Dollars	1,380	18,206
Oil Barrels (2)In Dollars	(1,380)	(18,206)
Natural gas MMBTUs In Dollars	44	490
Natural gas MMBTUs (2) In Dollars	$ (44)	$ (490)
Non-Hedging Commodity Options [Member] | Notional Amount [Member]
Derivative Instruments [Line Items]
Oil Barrels	366,000	2,920,000
Oil Barrels (2)	366,000	2,920,000
Natural gas MMBTUs	240,000	480,000
Natural gas MMBTUs (2)	240,000	480,000