Disclosures about Oil and Natural Gas Activities (Unaudited)--Standardized Measure of Discounted Future Net Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash flows	$ 11,275,090	$ 10,660,532	$ 9,690,171
Future production costs	(3,605,107)	(3,841,723)	(3,737,069)
Future development and abandonment costs	(1,726,955)	(1,506,585)	(1,172,786)
Future income tax provisions	(1,487,121)	(1,328,925)	(1,223,036)
Net future cash flows	4,455,907	3,983,299	3,557,280
Less effect of 10% discount factor	(2,561,207)	(2,239,527)	(2,033,599)
Standardized measure of discounted future net cash flows	$ 1,894,700	$ 1,743,772	$ 1,523,681	$ 1,597,912

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Amount of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for costs to be incurred in developing proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for income tax, based on the future income less future expense derived from production and sale of proved oil and gas reserves.

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Amount, after future accumulated cash outflow for development and production costs and future income tax, of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

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Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and gas reserves.

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Amount of discount on future accumulated cash inflow, after deducting future accumulated cash outflow for development and production costs and future income tax, from sale of proved oil and gas reserves, using a discount rate of ten percent a year.

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Amount of discounted future accumulated cash inflow relating to proved oil and gas reserves.

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