Disclosures about oil and natural gas activities (unaudited) (Tables)

12 Months Ended

Dec. 31, 2014

Oil and Gas Exploration and Production Industries Disclosures [Abstract]

Summary of changes in quantities of proved oil and natural gas reserves

Our oil and natural gas reserves are attributable solely to properties within the United States. A summary of the changes in our quantities of proved oil and natural gas reserves for the three years ended December 31, 2014 are as follows:

	Oil (MBbls)(1)		Natural Gas (MMcf)		Total (MBoe)
Proved developed and undeveloped reserves
Balance at January 1, 2012	100,380		335,280		156,260
Purchase of minerals in place	—		57		9
Sales of minerals in place	(2,694	)	(6,608	)	(3,795	)
Extensions and discoveries	7,117		37,256		13,326
Revisions (2)	3,410		(89,036	)	(11,429	)
Improved recoveries	842		—		842
Production	(5,812	)	(19,834	)	(9,118	)
Balance at December 31, 2012	103,243		257,115		146,095
Proved developed reserves at January 1, 2012	62,450		226,008		100,118
Proved developed reserves at December 31, 2012	63,956		185,826		94,927
Proved undeveloped reserves at January 1, 2012	37,930		109,272		56,142
Proved undeveloped reserves at December 31, 2012	39,287		71,289		51,169

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(1) Includes natural gas liquids.

(2) The downward revision in our reserves during 2012 was primarily due to a decrease in both oil and natural gas pricing used to estimate the reserves.

	Oil (MBbls)		Natural gas (MMcf)		Natural Gas Liquids (MBbls)		Total (MBoe)
Proved developed and undeveloped reserves
As of January 1, 2013	92,047		257,115		11,195		146,095
Purchase of minerals in place	3,283		3,618		1,059		4,945
Sales of minerals in place	(5,174	)	(9,708	)	(324	)	(7,116	)
Extensions and discoveries	11,556		62,412		3,127		25,085
Revisions (1)	(7,772	)	9,735		1,479		(4,671	)
Improved recoveries	3,879		—		—		3,879
Production	(5,006	)	(20,250	)	(1,361	)	(9,742	)
Balance at December 31, 2013	92,813		302,922		15,175		158,475
Purchase of minerals in place	276		859		55		474
Sales of minerals in place	(8,539	)	(79,579	)	(1,959	)	(23,761	)
Extensions and discoveries	12,776		56,159		3,405		25,541
Revisions(1)	(3,356	)	(11,957	)	1,741		(3,608	)
Improved recoveries	13,254		—		—		13,254
Production	(5,977	)	(20,648	)	(1,564	)	(10,982	)
Balance at December 31, 2014	101,247		247,756		16,853		159,393
Proved developed reserves:
December 31, 2013	56,360		196,920		11,484		100,664
December 31, 2014	54,862		158,265		11,787		93,027
Proved undeveloped reserves:
December 31, 2013	36,453		106,002		3,691		57,811
December 31, 2014	46,385		89,491		5,066		66,366

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(1) The downward revision in our reserves during 2014 was primarily due to removing proved undeveloped reserves that are not expected to be developed within the five-year time frame mandated by the SEC and a decline in the estimated sales margin on natural gas. The downward revision in our reserves during 2013 was primarily due to removing proved undeveloped reserves that are not expected to be developed within the five-year time frame and to negative technical revisions. The 2013 downward revisions were partially offset by positive revisions due to improved pricing of oil and natural gas.

Standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves

The standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves are as follows:

	For the year ended December 31,
	2014			2013			2012
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

Changes in standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves

The changes in the standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves are as follows:

	For the year ended December 31,
	2014			2013			2012
Beginning of year	$	1,743,772		$	1,523,681		$	1,597,912
Sale of oil and natural gas produced, net of production costs	(502,928		)	(418,224		)	(346,627		)
Net changes in prices and production costs	116,977			172,975			(206,640		)
Extensions and discoveries	286,500			514,743			224,903
Improved recoveries	148,673			79,589			14,204
Changes in future development costs	(91,027		)	(236,121		)	(18,184		)
Development costs incurred during the period that reduced future development costs	316,490			110,354			118,502
Revisions of previous quantity estimates	(40,481		)	(95,855		)	(192,894		)
Purchases and sales of reserves in place, net	(278,825		)	(20,021		)	(54,070		)
Accretion of discount	223,324			152,830			214,794
Net change in income taxes	(46,584		)	(60,981		)	166,238
Changes in production rates and other	18,809			20,802			5,543
End of year	$	1,894,700		$	1,743,772		$	1,523,681

Prices used in determining future net revenues related to standardized measure calculation

The following prices for oil, natural gas, and natural gas liquids before field differentials were used in determining future net revenues related to the standardized measure calculation.

	2014		2013		2012
Oil (per Bbl)	$	94.99	$	96.78	$	94.71
Natural gas (per Mcf)	$	4.35	$	3.67	$	2.76
Natural gas liquids (per Bbl) (1)	$	36.10	$	32.53	$	—

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(1) The NGL price is provided only for 2014 and 2013 to coincide with our separate presentation of NGL reserves in those years.