AngularJS Function After Multiple Async Service Calls

I have a REST API that I want to call from an AngularJS service like this:

angular.module('myModule').service('MyApi', ['$http', function($http) {
    return ({
        resources: resources,
        details: details
    });

    function resources() {
        return $http.jsonp('/api/resources');
    }

    function details(key) {
        return $http.jsonp('/api/details/' + id);
    }
}]);

There are other implementation details removed there such as authentication that aren't important. The API is provided by a third party so I can't change it.

GET /api/resources returns something like:

[{ "key": "one" }, { "key": "two" }]

GET /api/details/one returns something like:

{ "count": 5 }

I then have a controller where I want to call MyApi.resources(), wait for the results and then for each result, call MyApi.details(resource). When the final call to MyApi.details(resource) completes, I want to run a function to aggregate some results from the set of details, but I can't work out how to trigger this at the end.

My controller currently looks like this:

angular.module('myModule').controller('MyCtrl', ['$scope', 'MyApi', function($scope, MyApi) {
    $scope.results = new Array();

    MyApi.resources().then(function(response) {
        var resources = response.data;
        var length = resources.length;

        for (var i = 0; i < length; i++) {
            MyApi.details(resources[i].key).then(function(response) {
                $scope.results.push(response.data.count);
            });
        }
    });

    // how do I get this line to run only after all the results are returned?
    $scope.total = $scope.results.reduce(function(a, b) { return a + b; }, 0);
}]);

What is the best way to achieve the aggregation at the end?