pre_text: Year Ended December 31, 2018 Compared to Year Ended December 31, 2017 Revenue Total revenue for the year ended December 31, 2018 increased by $58.9 million, or 28%, compared to the year ended December 31, 2017. Subscription
  and returns revenue for the year ended December 31, 2018 increased by $54.1 million, or 27%, compared to the year ended December 31, 2017. Professional
  services revenue for the year ended December 31, 2018 increased by $4.8 million, or 37%, compared to the year ended December 31, 2017. Growth in total
  revenue was due primarily to increased demand for our products and services from both new and existing customers. Of the increase in total revenue for the year
  ended December 31, 2018 compared to 2017, approximately $29.6 million was attributable to existing customers, approximately $28.3 million was attributable to
  new customers, and approximately $1.1 million was due to interest income on funds held for customers. Total subscription and returns revenue for 2018 included
  $1.2 million related to our cross-border transactions technology acquired in May 2018.
table: For the Year Ended December 31, Change
table: 2018 2017 Amount Percentage
table: (dollars in thousands)
table: Revenue:
table: Subscription and returns $254,056 $199,942 $54,114 27%
table: Professional services 18,042 13,217 4,825 37%
table: Total revenue $272,098 $213,159 $58,939 28%
question: Analyse this data from a financial earnings document. What is the value of revenue from professional services as a percentage of the 2018 total revenue?