vue.js watch array variable within v-repeat

I am trying to watch a variable within an array that is bound to a repeating element using v-repeat but it doesnt seem to be working. There is a fiddle here

My HTML looks something like this

<div id="test">
    <div v-repeat="details">
        <select v-model="hour">
            <option value="1">1</option>
            <option value="2">2</option>
            <option value="3">3</option>
        </select>
    </div>
    <pre>
          {{ $data | json }}
    </pre>
</div>

and the vue js like this

new Vue({
        el: '#test',
        data: {
            details: [
                { 
                    hour: 2,
                    changeThis: null
                },
                { 
                    hour: 3,
                    changeThis: null
                }
            ],
        },
        watch: {
            'details': function (val, oldVal) {
                alert()
            },
        }
})

The binding to the hour properties is working fine, but I cannot get the watch to fire. I have also tried deep: true but that doesnt work either. If I define my watch as

watch: {
    'details[0].hour' 

then I can get it to work, but as I dont know how many details I will have then I dont really want to do this for each one. What I ideally want to do is get a reference to the array item that is being updated and then go from there. I could possibly achieve what I want by adding a

v-on="change:someVar(detail)" 

to the select element and doing it that way but I'm curious as to why the approach I am trying does not work.

Edit It is actually possible to return the full array by using the deep watcher as @kishanterry notes below (thank you for that), I had overlooked this syntax in the docs.. However it still doesn't quite give the solution I was hoping for

// deep watcher
    'c': {
      handler: function (val, oldVal) { /* ... */ },
      deep: true
    }