Table:
|  | December 31, 2019 | December 31, 2018 |
| --- | --- | --- |
|  | (Dollars in millions) |  |
| Customer receivables(1) | $2,194 | 2,346 |
| Contract liabilities | 1,028 | 860 |
| Contract assets | 130 | 140 |
Paragraphs:
Customer Receivables and Contract Balances
The following table provides balances of customer receivables, contract assets and contract liabilities as of December 31, 2019 and December 31, 2018:
(1) Gross customer receivables of $2.3 billion and $2.5 billion, net of allowance for doubtful accounts of $94 million and $132 million, at December 31, 2019 and December 31, 2018, respectively.
Contract liabilities are consideration we have received from our customers or billed in advance of providing goods or services promised in the future. We defer recognizing this consideration as revenue until we have satisfied the related performance obligation to the customer. Contract liabilities include recurring services billed one month in advance and installation and maintenance charges that are deferred and recognized over the actual or expected contract term, which typically ranges from one to five years depending on the service. Contract liabilities are included within deferred revenue in our consolidated balance sheet. During the years ended December 31, 2019 and December 31, 2018, we recognized $630 million and $295 million, respectively, of revenue that was included in contract liabilities as of January 1, 2019 and January 1, 2018, respectively.
Question:
What do contract liabilities include?
Answer: recurring services billed one month in advance and installation and maintenance charges that are deferred and recognized over the actual or expected contract term, which typically ranges from one to five years depending on the service