Oil and natural gas activities (Tables)

12 Months Ended

Dec. 31, 2014

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:

	2014		2013		2012
Property acquisition costs
Proved properties	$	3,496	$	69,962	$	1,108
Unproved properties	84,938		145,853		46,895
Total acquisition costs	88,434		215,815		48,003
Development costs	561,578		376,394		409,429
Exploration costs(1)	90,146		77,507		54,432
Total	$	740,158	$	669,716	$	511,864

 ________________

(1) Includes $152, $37,344, and $52,188 of EOR costs in 2014, 2013, and 2012, respectively.