var currentPage = 1;
List<Result> idList = new List<Result>();

var response = this.GetDocuments(currentPage).Result;
//idList.AddRange(response.results);

var tasks = new List<Task>();

for (var i = 0; i < response.meta.page.total_pages; ++i)
{
    var t = new Task(() =>
    {
        response = this.GetDocuments(i+1).Result;
        idList.AddRange(response.results);
    });
    tasks.Add(t);
    t.Start();
}

Task.WhenAll(tasks).ConfigureAwait(false);