Proved reserves (Details 6) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 MBoe	Dec. 31, 2011 MBoe	Dec. 31, 2010 MBoe
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, energy	69,131	79,460	82,404
Proved undeveloped reserves, energy	11,355	18,191	25,195
Balance at end of year, energy	80,486	97,651	107,599
Changes in proved and undeveloped reserves [Abstract]
Change in PUD reserve	(6,800)
Amount of PUD reserves converted into proved developed reserves	3,900
Negative revision to PUD reserves	10,300
New PUD reserves	7,400
Drilling, completion and future development costs [Abstract]
Drilling and completion capital	$ 58.4
Future development costs estimated to be spent to develop PUD reserves year one	147.5
Future development costs estimated to be spent to develop PUD reserves year two	24.3
Future development costs estimated to be spent to develop PUD reserves year three	$ 12.0
Oil (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	27,412	23,653	22,352
Proved undeveloped reserves, volume	6,041	3,352	3,314
Balance at end of year, volume	33,453	27,005	25,666
NGL (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	5,342	5,225	4,234
Proved undeveloped reserves, volume	1,811	2,117	2,967
Balance at end of year, volume	7,153	7,342	7,201
Natural Gas (MMcf)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	218,259	303,495	334,911
Proved undeveloped reserves, volume	21,019	76,332	113,486
Balance at end of year, volume	239,278	379,827	448,397