Why "long" filed can save double value?

I create a index ,and create a filed,and this field's data type is long, I try to save double value in this field,and I think ES will change data type to double,but nothing happen, someone can tell me why?

this is filed mapping

purchase.latest_priceHk: {
type: "long"

this is value

purchase.latest_priceHk: 147.83