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Supplemental Oil and Gas Reserve Information - Unaudited (Tables)
 12 Months Ended
Dec. 31, 2017
Oil and Gas Exploration and Production Industries Disclosures [Abstract]  
Net Proved, Proved Developed and Proved Undeveloped Reserves
The following table sets forth the Company’s estimated net proved, proved developed and proved undeveloped reserves at December 31, 2017, 2016 and 2015:
 
 
Oil
(MBbl)
 
Gas
(MMcf)
 
MBoe
2015
 
 
 
 
 
Proved reserves
 
 
 
 
 
Beginning balance
235,399

 
220,120

 
272,086

Revisions of previous estimates
(75,458
)
 
(55,065
)
 
(84,635
)
Extensions, discoveries and other additions
38,962

 
46,072

 
46,640

Sales of reserves in place
—

 
—

 
—

Purchases of reserves in place
2,115

 
2,702

 
2,565

Production
(16,090
)
 
(14,001
)
 
(18,423
)
Net proved reserves at December 31, 2015
184,928

 
199,828

 
218,233

Proved developed reserves, December 31, 2015
127,445

 
120,789

 
147,577

Proved undeveloped reserves, December 31, 2015
57,483

 
79,039

 
70,656

2016
 
 
 
 
 
Proved reserves
 
 
 
 
 
Beginning balance
184,928

 
199,828

 
218,233

Revisions of previous estimates
11,713

 
116,539

 
31,136

Extensions, discoveries and other additions
10,790

 
24,520

 
14,876

Sales of reserves in place
(5,828
)
 
(10,839
)
 
(7,635
)
Purchases of reserves in place
50,164

 
100,629

 
66,936

Production
(15,174
)
 
(19,573
)
 
(18,436
)
Net proved reserves at December 31, 2016
236,593

 
411,104

 
305,110

Proved developed reserves, December 31, 2016
152,337

 
229,568

 
190,598

Proved undeveloped reserves, December 31, 2016
84,256

 
181,536

 
114,512

2017
 
 
 
 
 
Proved reserves
 
 
 
 
 
Beginning balance
236,593

 
411,104

 
305,110

Revisions of previous estimates
(28,323
)
 
54,726

 
(19,200
)
Extensions, discoveries and other additions
36,238

 
89,489

 
51,153

Sales of reserves in place
(1,196
)
 
(1,147
)
 
(1,387
)
Purchases of reserves in place
466

 
1,230

 
671

Production
(18,818
)
 
(31,946
)
 
(24,143
)
Net proved reserves at December 31, 2017
224,960

 
523,456

 
312,204

Proved developed reserves, December 31, 2017
150,628

 
301,101

 
200,812

Proved undeveloped reserves, December 31, 2017
74,332

 
222,355

 
111,392
Standardized Measure of Discounted Future Cash Flows
The following table sets forth the Standardized Measure of discounted future net cash flows from projected production of the Company’s estimated net proved reserves at December 31, 2017, 2016 and 2015:
 
At December 31,
 
2017
 
2016
 
2015
 
(In thousands)
Future cash inflows
$
11,636,126

 
$
9,426,963

 
$
8,582,831

Future production costs
(4,458,418
)
 
(3,996,657
)
 
(3,842,517
)
Future development costs
(992,271
)
 
(784,727
)
 
(909,562
)
Future income tax expense
(580,481
)
 
(279,345
)
 
(225,662
)
Future net cash flows
5,604,956

 
4,366,234

 
3,605,090

10% annual discount for estimated timing of cash flows
(2,304,261
)
 
(1,883,169
)
 
(1,690,760
)
Standardized measure of discounted future net cash flows
$
3,300,695

 
$
2,483,065

 
$
1,914,330
Changes in Standardized Measure of Discounted Future Net Cash Flows Applicable to Proved Oil and Natural Gas Reserves
The following table sets forth the changes in the Standardized Measure of discounted future net cash flows applicable to estimated net proved reserves for the periods presented:
 
2017
 
2016
 
2015
 
(In thousands)
January 1
$
2,483,065

 
$
1,914,330

 
$
3,981,661

Net changes in prices and production costs
881,993

 
(367,527
)
 
(3,201,195
)
Net changes in future development costs
(60,929
)
 
69,992

 
150,333

Sales of oil and natural gas, net
(769,625
)
 
(403,739
)
 
(477,755
)
Extensions
661,467

 
165,926

 
409,838

Discoveries
—

 
—

 
—

Purchases of reserves in place
6,518

 
533,505

 
14,378

Sales of reserves in place
(9,024
)
 
(57,770
)
 
—

Revisions of previous quantity estimates
(78,942
)
 
333,398

 
(946,729
)
Previously estimated development costs incurred
157,386

 
91,518

 
216,981

Accretion of discount
262,776

 
202,272

 
548,141

Net change in income taxes
(238,354
)
 
(36,303
)
 
1,391,358

Changes in timing and other
4,364

 
37,463

 
(172,681
)
December 31
$
3,300,695

 
$
2,483,065

 
$
1,914,330