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Proved reserves (Details 5) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
MBoe
Dec. 31, 2012
MBoe
Dec. 31, 2011
MBoe
Proved Developed and Undeveloped Reserves [Abstract]      
Proved developed reserves, energy 66,140 69,131 79,460
Proved undeveloped reserves, energy 14,555 11,355 18,191
Balance at end of year, energy 80,695 80,486 97,651
Changes in proved and undeveloped reserves [Abstract]      
Change in PUD reserve 3,200    
Negative revision to PUD reserves (1,600)    
New PUD reserves 11,900    
Converting PUD reserves 7,100    
Drilling, completion and future development costs [Abstract]      
Drilling and completion capital $ 127.3    
Future development costs estimated to be spent to develop PUD reserves year one 143.6    
Future development costs estimated to be spent to develop PUD reserves year two 116.0    
Future development costs estimated to be spent to develop PUD reserves year three $ 18.1    
Oil (MBbls)
      
Proved Developed and Undeveloped Reserves [Abstract]      
Proved developed reserves, volume 31,394 27,412 23,653
Proved undeveloped reserves, volume 9,625 6,041 3,352
Balance at end of year, volume 41,019 33,453 27,005
NGL (MBbls)
      
Proved Developed and Undeveloped Reserves [Abstract]      
Proved developed reserves, volume 5,322 5,342 5,225
Proved undeveloped reserves, volume 1,280 1,811 2,117
Balance at end of year, volume 6,602 7,153 7,342
Natural Gas (MMcf)
      
Proved Developed and Undeveloped Reserves [Abstract]      
Proved developed reserves, volume 176,546 218,259 303,495
Proved undeveloped reserves, volume 21,899 21,019 76,332
Balance at end of year, volume 198,445 239,278 379,827