pre_text: Accounts receivable and contract balances The timing of revenue recognition may differ from the time of billing to customers. Receivables presented in the balance sheet represent an unconditional right to consideration. Contract balances represent amounts from an arrangement when either the performance obligation has been satisfied by transferring goods and/or services to the customer in advance of receiving all or partial consideration for such goods and/or services from the customer, or the customer has made payment in advance of obtaining control of the goods and/or services promised to the customer in the contract. Contract assets primarily relate to rights to consideration for goods and/or services provided to the customers but for which there is not an unconditional right at the reporting date. Under a fixed-term plan, the total contract revenue is allocated between wireless services and equipment revenues, as discussed above. In conjunction with these arrangements, a contract asset is created, which represents the difference between the amount of equipment revenue recognized upon sale and the amount of consideration received from the customer. The contract asset is recognized as accounts receivable as wireless services are provided and billed. The right to bill the customer is
  
  obtained as service is provided over time, which results in the right to the payment being unconditional. The contract asset balances are presented in the balance sheets as prepaid expenses and other, and other assets - net. Contract assets are assessed for impairment on an annual basis and an impairment charge is recognized to the extent the carrying amount is not recoverable. The impairment charge related to contract assets was insignificant for the years ended December 31, 2019 and 2018. Increases in the contract asset balances were primarily due to new contracts and increases in sales promotions recognized upfront, driven by customer activity related to wireless services, while decreases were due to reclassifications to accounts receivable due to billings on the existing contracts and insignificant impairment charges. Contract liabilities arise when customers are billed and consideration is received in advance of providing the goods and/or services promised in the contract. The majority of the contract liability at each year end is recognized during the following year as these contract liabilities primarily relate to advanced billing of fixed monthly fees for service that are recognized within the following month when services are provided to the customer. The contract liability balances are presented in the balance sheet as contract liabilities and other, and other liabilities. Increases in contract liabilities were primarily due to increases in sales promotions recognized over time and upfront fees, as well as increases in deferred revenue related to advanced billings, while decreases in contract liabilities were primarily due to the satisfaction of performance obligations related to wireless services. The balance of receivables from contracts with customers, contract assets and contract liabilities recorded in the balance sheet were as follows: (1) Balances do not include receivables related to the following contracts: leasing arrangements (such as towers) and the interest on equipment financed on a device payment plan agreement when sold to the customer by an authorized agent. (2) Included in device payment plan agreement receivables presented in Device Payment Plans Note. Balances do not include receivables related to contracts completed prior to January 1, 2018 and receivables derived from the sale of equipment on a device payment plan through an authorized agent.
table: At December 31, 2019 At December 31, 2018 At January 1, 2018
table: Receivables (1) $ 5,752 $ 5,448 $ 5,555
table: Device payment plan agreement receivables (2) 15,313 12,272 2,073
table: Contract assets 761 772 858
table: Contract liabilities 4,721 4,521 3,445
question: Analyse this data from a financial earnings document. What was the increase / (decrease) in contract liabilities from December 2018 to December 2019?