pre_text: Restricted Stock Units A summary of the RSU transactions for the year ended December 31, 2019 are as follows (number of shares in millions): During 2019, the Company awarded 2.6 million RSUs to certain officers and employees of the Company that vest upon the achievement of certain performance criteria and market conditions. The number of units expected to vest is evaluated each reporting period and compensation expense is recognized for those units for which achievement of the performance criteria is considered probable. Compensation expense for RSUs with market conditions are recognized based on the grant date fair value irrespective of the achievement of the condition. As of December 31, 2019, unrecognized compensation expense, net of estimated forfeitures related to non-vested RSUs granted under the Amended and Restated SIP with service, performance and market conditions, was $60.9 million, $10.1 million and $3.9 million, respectively. For RSUs with time-based service conditions, expense is being recognized over the vesting period; for RSUs with performance criteria, expense is recognized over the period during which the performance criteria is expected to be achieved; for RSUs with market conditions expense is recognized over the period in which the condition is assessed irrespective of whether it would be achieved or not. Unrecognized compensation cost related to awards with certain performance criteria that are not expected to be achieved is not included here. Total compensation expense related to performance-based, service-based, and market-based RSUs was $69.8 million for the year ended December 31, 2019, which included $48.4 million for RSUs with time-based service conditions that were granted in 2019 and prior that are expected to vest.
table: Number of Shares Weighted-Average Grant
  Date Fair Value
table: Nonvested shares of RSUs at December 31, 2018 8.6 $16.59
table: Granted 5.4 21.64
table: Achieved 0.2 24.46
table: Released (4.8) 14.41
table: Canceled (0.5) 19.74
table: Nonvested shares of RSUs at December 31, 2019 8.9 20.84
question: Analyse this data from a financial earnings document. What is the average Number of Nonvested shares of RSUs for December 31, 2018 to 2019?