Non-Mineral Leases - Operating lease rental (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Operating lease rental expense
Total rentals	$ 282	[1]	$ 304	[1]	$ 267	[1]
Less sublease rentals	(15)		(14)		(14)
Operating Leases, Rent Expense, Total	267		290		253
Non Mineral Leases (Textual) [Abstract]
Contingent Rentals	$ 3		$ 29		$ 16

[1] *Includes $3 million, $29 million and $16 million of contingent rentals in 2012, 2011 and 2010, respectively. Contingent rentals primarily are related to drilling equipment and are based on usage or volume of product sold.