Property and Equipment

The historical costs of our property and equipment and related accumulated depreciation balances were as follows at the dates indicated (in thousands):



	Estimated
	Useful Life	December 31,
	in Years	2017			2016

Tractors and trailers ⁽¹⁾	5 – 6	$	88,065		$	89,576
Oil and gas (successful efforts)		—			62,784
Field equipment	2 – 5	18,490			18,282
Buildings	5 – 39	15,727			15,707
Office equipment	1 – 5	1,929			1,913
Land		1,790			1,790
Construction in progress		275			596
Total		126,276			190,648
Less accumulated depreciation		(96,914		)	(144,323		)
Property and equipment, net		$	29,362		$	46,325

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(1)	2017 includes assets held under capital leases. During the third quarter of 2017, we entered into capital leases for certain tractors in our marketing segment. Gross property and equipment and accumulated amortization associated with assets held under capital leases were $1.8 million and $0.1 million, respectively, at December 31, 2017 (see Note 13 for further information).

Components of depreciation, depletion and amortization expense were as follows for the periods indicated (in thousands):



	Year Ended December 31,
	2017		2016		2015
Depreciation, depletion and amortization, excluding amounts
under capital leases	$	13,478	$	18,792	$	23,717
Amortization of property and equipment under capital leases	121		—		—
Total depreciation, depletion and amortization	$	13,599	$	18,792	$	23,717

Crude Oil and Natural Gas Exploration and Production Assets

Our subsidiary that owned the upstream crude oil and natural gas exploration and production assets was deconsolidated effective with its bankruptcy filing in April 2017 and subsequently accounted for as a cost method investment (see Note 3). These upstream crude oil and natural gas exploration and production assets were sold during the third quarter of 2017. We have no further interest in these assets.

Impairment provisions including in upstream crude oil and natural gas exploration and production segment operating losses were as follows for the periods indicated (in thousands):



	Year Ended December 31,
	2017		2016		2015

Producing property impairments	$	—	$	30	$	10,324
Non-producing property impairments	3		283		1,758
Total crude oil and natural gas impairments	$	3	$	313	$	12,082

At December 31, 2017 and 2016, we had no capitalized costs for non-producing crude oil and natural gas leasehold interests.

Gains on sales of assets

We sold certain used trucks and equipment from our marketing and transportation segments and recorded net pre-tax gains as follows for the periods indicated (in thousands):



	Year Ended December 31,
	2017		2016		2015

Sales of used trucks and equipment	$	594	$	1,966	$	535

Asset Retirement Obligations

We record AROs for the estimated retirement costs associated with certain tangible long-lived assets. The estimated fair value of AROs are recorded in the period in which they are incurred and the corresponding cost capitalized by increasing the carrying amount of the related long-lived asset. The liability is accreted to its then present value each period, and the capitalized cost is depreciated over the useful life of the asset. If the liability is settled for an amount other than the recorded amount, an increase or decrease to expense is recognized. A summary of our AROs is presented as follows for the periods indicated (in thousands):



	Year Ended December 31,
	2017			2016			2015

ARO liability beginning balance	$	2,329		$	2,469		$	2,464
Liabilities incurred	18			162			39
Accretion of discount	58			92			93
Liabilities settled	(261		)	(394		)	(127		)
Deconsolidation of subsidiary ⁽¹⁾	(871		)	—			—
ARO liability ending balance	$	1,273		$	2,329		$	2,469

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(1)	Relates to our upstream crude oil and natural gas exploration and production subsidiary that was deconsolidated in April 2017 as a result of its bankruptcy filing (see Note 3 for further information).