financial assurance we must provide financial assurance to governmental agencies and a variety of other entities under applicable environmental regulations relating to our landfill operations for capping , closure and post-closure costs , and related to our performance under certain collection , landfill and transfer station contracts . we satisfy these financial assurance requirements by providing surety bonds , letters of credit , or insurance policies ( financial assurance instruments ) , or trust deposits , which are included in restricted cash and marketable securities and other assets in our consolidated balance sheets . the amount of the financial assurance requirements for capping , closure and post-closure costs is determined by applicable state environmental regulations . the financial assurance requirements for capping , closure and post-closure costs may be associated with a portion of the landfill or the entire landfill . generally , states require a third-party engineering specialist to determine the estimated capping , closure and post-closure costs that are used to determine the required amount of financial assurance for a landfill . the amount of financial assurance required can , and generally will , differ from the obligation determined and recorded under u.s . gaap . the amount of the financial assurance requirements related to contract performance varies by contract . additionally , we must provide financial assurance for our insurance program and collateral for certain performance obligations . we do not expect a material increase in financial assurance requirements during 2018 , although the mix of financial assurance instruments may change . these financial assurance instruments are issued in the normal course of business and are not considered indebtedness . because we currently have no liability for the financial assurance instruments , they are not reflected in our consolidated balance sheets ; however , we record capping , closure and post-closure liabilities and insurance liabilities as they are incurred . off-balance sheet arrangements we have no off-balance sheet debt or similar obligations , other than operating leases and financial assurances , which are not classified as debt . we have no transactions or obligations with related parties that are not disclosed , consolidated into or reflected in our reported financial position or results of operations . we have not guaranteed any third-party debt . free cash flow we define free cash flow , which is not a measure determined in accordance with u.s . gaap , as cash provided by operating activities less purchases of property and equipment , plus proceeds from sales of property and equipment , as presented in our consolidated statements of cash flows . the following table calculates our free cash flow for the years ended december 31 , 2017 , 2016 and 2015 ( in millions of dollars ) : .
what is the average from the proceeds from sales of property and equipment from 2015 to 2017
for a discussion of the changes in the components of free cash flow , see our discussion regarding cash flows provided by operating activities and cash flows used in investing activities contained elsewhere in this management 2019s discussion and analysis of financial condition and results of operations. .