pre_text: (15) CONTRACT BALANCES Contract balances at December 31 are set forth in the following table: The change in our net contract assets/(liabilities) from December 31, 2018 to December 31, 2019 was due primarily to the timing of payments and invoicing relating to SaaS and PCS renewals, partially offset by revenues recognized in the year ended December 31, 2019 of $674.2, related to our contract liability balances at December 31, 2018. In addition, the impact of the 2019 business acquisitions increased net contract liabilities by $96.2. In order to determine revenues recognized in the period from contract liabilities, we allocate revenue to the individual deferred revenue or BIE balance outstanding at the beginning of the year until the revenue exceeds that balance. Impairment losses recognized on our accounts receivable and unbilled receivables were immaterial in the year ended December 31, 2019. (1) Consists of “Deferred revenue,” and billings in-excess of revenues (“BIE”). BIE are reported in “Other accrued liabilities” in our Consolidated Balance Sheets.
table: Balance Sheet Account 2019 2018 Change
table: Unbilled receivables $  183.5 $  169.4 $    14.1
table: Contract liabilities - current(1) (840.8) (714.1) (126.7)
table: Deferred revenue - non-current (33.2) (29.8) (3.4)
table: Net contract assets/(liabilities) $ (690.5) $ (574.5) $ (116.0)
question: Analyse this data from a financial earnings document. What are the average unbilled receivables from 2018 to 2019?