Rental Activity

We rent natural gas compressor packages to entities in the petroleum industry. These rental arrangements are classified as operating leases and generally have original terms of six months to twenty-four months and continue on a month-to-month basis thereafter. Depreciation expense for rental equipment was $20.0 million, $20.2 million and $21.0 million for the years ended December 31, 2017, 2016 and 2015, respectively. Future minimum rent payments for arrangements not on a month-to-month basis at December 31, 2017 are as follows :



Years Ending December 31, (in thousands)
2018	$	6,494
2019	976
2020	543
2021	542
2022	542
Total	$	9,097