Amazon defines this as:
Includes annual and monthly fees associated with Amazon Prime membership, as well as audiobook, e-book, digital video, digital music, and other subscription services.
2014: $2.762 billion, 2015: $4.467 billion, 2016: $6.394 billion.
Last reported quarter 2017 Q3 it was $2.44 billion, up by 59% from $1.53 billion the same quarter a year before.
Since 2015 it increased by 45%, from $4.42 billion to $6.39 billion in 2016.
Source: Amazon Quarterly Results.