Oil and natural gas activities (Tables)

12 Months Ended

Dec. 31, 2013

Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Abstract]

Cost incurred in oil and natural gas producing activities

Our oil and natural gas activities are conducted entirely in the United States. Costs incurred in oil and natural gas producing activities are as follows for the years ended December 31:

	2013		2012		2011
Property acquisition costs
Proved properties	$	69,962	$	1,108	$	1,024
Unproved properties	145,853		46,895		15,795
Total acquisition costs	215,815		48,003		16,819
Development costs	376,394		409,429		250,182
Exploration costs(1)	77,507		54,432		57,016
Total	$	669,716	$	511,864	$	324,017

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(1) Includes $37,344, $52,188, and $33,030 of EOR costs in 2013, 2012, and 2011, respectively.