Supplemental oil and natural gas disclosures (Tables)

12 Months Ended

Dec. 31, 2013

Oil and Gas Exploration and Production Industries Disclosures [Abstract]

Schedule of costs incurred in the acquisition and development of oil and natural gas assets

Costs incurred in the acquisition and development of oil and natural gas assets are presented below for the periods presented:

	For the years ended December 31,
(in thousands)	2013		2012		2011
Property acquisition costs:
Proved	$	9,652	$	16,925	$	—
Unproved	27,087		3,693		—
Exploration	48,763		93,266		62,888
Development costs(1)	654,452		839,118		660,922
Total costs incurred	$	739,954	$	953,002	$	723,810

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(1)The costs incurred for oil and natural gas development activities include $6.8 million, $7.4 million and $4.5 million, in asset retirement obligations for the years ended December 31, 2013, 2012 and 2011, respectively.

Schedule of aggregate capitalized costs related to oil and natural gas production activities with applicable accumulated depreciation, depletion, amortization and impairment

Aggregate capitalized costs related to oil and natural gas production activities with applicable accumulated depletion, depreciation and impairment are presented below for the periods presented:

	For the years ended December 31,
(in thousands)	2013		2012		2011
Capitalized costs:
Proved properties	$	3,276,578	$	2,993,266	$	2,083,015
Unproved properties	208,085		159,946		117,195
	3,484,663		3,153,212		2,200,210
Less accumulated depletion, depreciation and impairment	1,349,315		1,121,273		884,533
Net capitalized costs	$	2,135,348	$	2,031,939	$	1,315,677

Summary of oil and natural gas property costs not being amortized by year

The following table shows a summary of the oil and natural gas property costs not being amortized as of December 31, 2013, by year in which such costs were incurred:

(in thousands)	2013		2012		2011		2010 and prior		Total
Unproved properties	$	124,588	$	67,370	$	8,249	$	7,878	$	208,085

Summary of results of oil and natural gas producing activities (excluding corporate overhead and interest costs)

The results of operations of oil and natural gas producing activities (excluding corporate overhead and interest costs) are presented below for the periods presented:

	For the years ended December 31,
(in thousands)	2013		2012		2011
Revenues:
Oil and natural gas sales	$	664,844	$	583,569	$	506,255
Production costs:
Lease operating expenses	79,136		67,325		43,306
Production and ad valorem taxes	42,396		37,637		31,982
	121,532		104,962		75,288
Other costs:
Depletion and depreciation	227,992		237,130		171,517
Accretion of asset retirement obligation	1,475		1,200		616
Income tax expense(1)	112,984		83,686		93,180
Results of operations	$	200,861	$	156,591	$	165,654

__________________________________________________________________________

(1)Income tax expense above is computed utilizing the statutory rate.

Schedule of analysis of change in estimated quantities of oil and natural gas reseves located within United States

The following table provides an analysis of the change in estimated quantities of oil and natural gas reserves, all of which are located within the United States, for the periods presented.

	Year ended December 31, 2013
	Oil (MBbl)		Gas (MMcf)		MBOE
Proved developed and undeveloped reserves:
Beginning of year	98,141		542,946		188,632
Revisions of previous estimates	(17,956	)	15,710		(15,338	)
Extensions, discoveries and other additions	37,850		192,229		69,888
Purchases of reserves in place	170		1,454		412
Sales of reserves in place	(1,220	)	(165,289	)	(28,768	)
Production	(5,487	)	(34,348	)	(11,211	)
End of year	111,498		552,702		203,615
Proved developed reserves:
Beginning of year	33,316		289,045		81,490
End of year	37,878		203,082		71,725
Proved undeveloped reserves:
Beginning of year	64,825		253,901		107,142
End of year	73,620		349,620		131,890

	Year ended December 31, 2012
	Oil (MBbl)		Gas (MMcf)		MBOE
Proved developed and undeveloped reserves:
Beginning of year	56,267		601,117		156,453
Revisions of previous estimates	(12,396	)	(260,651	)	(55,837	)
Extensions, discoveries and other additions	57,391		232,418		96,127
Purchases of reserves in place	1,654		9,210		3,189
Production	(4,775	)	(39,148	)	(11,300	)
End of year	98,141		542,946		188,632
Proved developed reserves:
Beginning of year	21,762		248,598		63,195
End of year	33,316		289,045		81,490
Proved undeveloped reserves:
Beginning of year	34,505		352,519		93,258
End of year	64,825		253,901		107,142

	Year ended December 31, 2011
	Oil (MBbl)		Gas (MMcf)		MBOE
Proved developed and undeveloped reserves:
Beginning of year	44,847		550,278		136,560
Revisions of previous estimates	(1,124	)	(47,296	)	(9,006	)
Extensions, discoveries and other additions	15,912		129,846		37,553
Purchases of reserves in place	—		—		—
Production	(3,368	)	(31,711	)	(8,654	)
End of year	56,267		601,117		156,453
Proved developed reserves:
Beginning of year	12,420		194,481		44,833
End of year	21,762		248,598		63,195
Proved undeveloped reserves:
Beginning of year	32,427		355,797		91,727
End of year	34,505		352,519		93,258

Standardized measure of discounted future 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 is as follows for the periods presented:

	For the years ended December 31,
(in thousands)	2013			2012			2011
Future cash inflows	$	13,337,798		$	11,636,926		$	8,856,906
Future production costs	(3,059,368		)	(3,163,371		)	(2,562,237		)
Future development costs	(2,250,950		)	(2,252,559		)	(1,959,818		)
Future income tax expenses	(2,150,983		)	(1,433,373		)	(999,185		)
Future net cash flows	5,876,497			4,787,623			3,335,666
10% discount for estimated timing of cash flows	(3,554,293		)	(2,910,167		)	(1,934,807		)
Standardized measure of discounted future net cash flows	$	2,322,204		$	1,877,456		$	1,400,859

Schedule of changes in standardized measure of discounted future net cash flows

Changes in the standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves are as follows for the periods presented:

	For the years ended December 31,
(in thousands)	2013			2012			2011
Standardized measure of discounted future net cash flows, beginning of year	$	1,877,456		$	1,400,859		$	869,982
Changes in the year resulting from:
Sales, less production costs	(543,312		)	(478,607		)	(430,967		)
Revisions of previous quantity estimates	(190,961		)	(631,693		)	(70,021		)
Extensions, discoveries and other additions	1,166,481			1,287,952			529,041
Net change in prices and production costs	313,947			194,921			566,034
Changes in estimated future development costs	921			(3,917		)	(163,399		)
Previously estimated development costs incurred during the period	89,396			137,510			207,818
Purchases of reserves in place	7,604			25,041			—
Divestitures of reserves in place	(239,148		)	—			—
Accretion of discount	234,852			176,996			106,170
Net change in income taxes	(259,991		)	(101,955		)	(176,165		)
Timing differences and other	(135,041		)	(129,651		)	(37,634		)
Standardized measure of discounted future net cash flows, end of year	$	2,322,204		$	1,877,456		$	1,400,859