Note 15: Oil and natural gas 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:

 

		2016				2015				2014
Property acquisition costs
Proved properties		$	390			$	1,192			$	3,496
Unproved properties			15,497				24,735				84,938
Total acquisition costs			15,887				25,927				88,434
Development costs			114,472				150,261				561,578
Exploration costs			19,055				33,091				90,146
Total		$	149,414			$	209,279			$	740,158

Depreciation, depletion, and amortization expense of oil and natural gas properties was as follows for the years ended December 31:

 

		2016				2015				2014
DD&A		$	111,793			$	204,692			$	231,761
DD&A per BOE:		$	12.52			$	20.07			$	21.10

 

Oil and natural gas properties not subject to amortization consists of unevaluated leasehold acquisition costs, capitalized interest related to the leasehold costs and wells or facilities for which reserve volumes are not classified as proved until completed.

 

		Year incurred
		2016				2015				Total
Leasehold acquisitions		$	14,170			$	1,285			$	15,455
Capitalized interest			1,356				538				1,894
Wells and facilities in progress of completion			3,004				—				3,004
Total unevaluated oil and natural gas properties excluded from amortization		$	18,530			$	1,823			$	20,353

The wells and facilities in progress of completion were completed in 2017 and transferred to the amortization base while we expect to complete our evaluation for the majority of the leasehold acreage costs within the next two to five years.