Oil and natural gas activities

12 Months Ended

Dec. 31, 2014

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

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

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

Depreciation, depletion, and amortization expense of oil and natural gas properties was $231,761, $179,734, and $154,788 for the years ended December 31, 2014, 2013, and 2012, respectively. The average depreciation, depletion and amortization rate per equivalent unit of production was $21.10, $18.45, and $16.98 for the years ended December 31, 2014, 2013, and 2012, respectively.

Oil and natural gas properties not subject to amortization consist of the cost of unevaluated properties and seismic costs associated with specific unevaluated properties. Of the $288,425 of unproved property costs at 2014 being excluded from the amortization base, $102,860, $122,494, and $62,092 were incurred in 2014, 2013, and 2012, respectively, and $979 was incurred in prior years. We expect to complete our evaluation for the majority of these costs relating to non-EOR properties within the next two to five years.