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Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Proved Developed and Undeveloped Reserves (Detail)
 12 Months Ended
Dec. 31, 2013
Mcf
Dec. 31, 2012
Mcf
Dec. 31, 2011
Mcf
Reserve Quantities [Line Items]      
Revisions 449,000,000 109,000,000 225,000,000
Extensions, discoveries and additions 1,700,000,000 1,800,000,000 1,500,000,000
Proved Undeveloped Reserves:      
Proved undeveloped reserves 4,009,608,000 3,048,068,000  
Natural Gas
      
Reserve Quantities [Line Items]      
Beginning Balance 4,792,676,000 4,009,676,000 3,566,526,000 [1]
Revisions 384,825,000 76,925,000 73,643,000
Extensions, discoveries and additions 853,746,000 996,059,000 1,304,324,000
Purchases         
Property sales (101,074,000) (73,429,000) (777,816,000)
Production (264,528,000) (216,555,000) (157,001,000)
Ending Balance 5,665,645,000 4,792,676,000 4,009,676,000
Proved Developed Reserves:      
Proved developed reserves 2,797,483,000 2,373,604,000 1,907,209,000
Proved Undeveloped Reserves:      
Proved undeveloped reserves 2,868,162,000 2,419,072,000 2,102,467,000
NGLs
      
Reserve Quantities [Line Items]      
Beginning Balance 240,399 142,515 122,722 [1]
Revisions 7,743 3,036 18,627
Extensions, discoveries and additions 135,810 113,392 26,591
Purchases         
Property sales (286) (11,575) (19,852)
Production (9,254) (6,969) (5,573)
Ending Balance 374,412 240,399 142,515
Proved Developed Reserves:      
Proved developed reserves 206,477 154,984 64,472
Proved Undeveloped Reserves:      
Proved undeveloped reserves 167,935 85,415 78,043
Crude Oil and Condensate
      
Reserve Quantities [Line Items]      
Beginning Balance 45,082 31,532 23,239 [1]
Revisions 2,935 2,316 6,522
Extensions, discoveries and additions 10,723 15,131 4,915
Purchases         
Property sales (6,553) (1,046) (1,176)
Production (3,827) (2,851) (1,968)
Ending Balance 48,360 45,082 31,532
Proved Developed Reserves:      
Proved developed reserves 26,054 25,667 17,872
Proved Undeveloped Reserves:      
Proved undeveloped reserves 22,306 19,415 13,660
Natural Gas Equivalents
      
Reserve Quantities [Line Items]      
Beginning Balance 6,505,570,000 5,053,961,000 4,442,290,000 [1]
Revisions 448,898,000 109,036,000 224,542,000
Extensions, discoveries and additions 1,732,944,000 1,767,202,000 1,493,357,000
Purchases         
Property sales (142,116,000) (149,153,000) (903,983,000)
Production (343,022,000) (275,476,000) (202,245,000)
Ending Balance 8,202,274,000 6,505,570,000 5,053,961,000
Proved Developed Reserves:      
Proved developed reserves 4,192,666,000 3,457,502,000 2,401,274,000
Proved Undeveloped Reserves:      
Proved undeveloped reserves 4,009,608,000 3,048,068,000 2,652,687,000
[1] Total proved reserves at December 31, 2010 includes 906,371 Mmcfe related to discontinued operations of which 408,710 Mmcfe is proved undeveloped.