Mongo DB Utilizing too much ram

MongoDB server continuously utilizing 15-16 GB ram of total 32 GB ram in windows enviornment.
There is continues selection and insertion operation is going on MongoDB collection which contains more than 200 million of documents.
So How we can reduce the ram utilization of MongoDB server ?

A fully utilised Wired Tiger cache would run to that.

Default cache size is (System RAM - 1GB) / 2